WASHINGTON (AP) — It's a major international mystery with global consequences: Who was behind the explosions that damaged the Nord Stream gas pipelines last year in the Baltic Sea?
The answer has broad implications for European energy security but could also threaten Western unity over backing Ukraine in defending itself from Russia's invasion. Or, it might shatter Russian and Chinese attempts to fix the blame on a hypocritical West.
Yet, nearly six months after the sabotage on the Russia-to-Germany pipelines, there is no accepted explanation. And a series of unconfirmed reports variously accusing Russia, the United States and Ukraine are filling an information vacuum as investigations into the blasts continue.
A look at the pipelines and what's known about the explosions.
WHAT ARE THE NORD STREAM PIPELINES?
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The pipelines, known as Nord Stream 1 and Nord Stream 2, are majority-owned by Russia's state-run energy giant Gazprom and used to transport natural gas from Russia to Europe under the Baltic to their termini in Germany.
Nord Stream 1 was completed and came online in 2011. Nord Stream 2 was not finished until the fall of 2021 but never became operational due to the launch of Russia's invasion of Ukraine in February 2022.

FILE - In this picture provided by Swedish Coast Guard, a leak from Nord Stream 2 is seen, on Sept. 28, 2022. Germany’s defense minister voiced caution Wednesday March 8, 2023 over media reports that a pro-Ukraine group was involved in blowing up the Nord Stream gas pipelines in the Baltic Sea last year. (Swedish Coast Guard via AP, File)
WHY ARE THEY CONTROVERSIAL?
Both pipelines bypass existing routes that go through Ukraine, meaning not only that Ukraine loses income from transit fees but is unable to directly use the gas they carry.
Of perhaps greater concern to the West, the pipelines were seen as a move by Russia to gain further, if not almost complete, control over Europe's energy supplies. Many in the West fear that Russia will use energy as a political weapon against European countries as it has done in the past with former Soviet states.
Despite those concerns and over the objections of the Obama, Trump and Biden administrations, the German government under former Chancellor Angela Merkel moved ahead with the construction of the Nord Stream 2 project. The Biden administration waived sanctions against German entities involved in Nord Stream 2 after securing a pledge from Germany that it would allow backflows of gas into Ukraine and would act to shut the pipeline down should Russia try to use it to force political concessions.
After Russia's Feb. 24, 2022, invasion of Ukraine, Germany withdrew permission for Nord Stream 2, which had not yet come online.

FILE - Pipes at the landfall facilities of the 'Nord Stream 2' gas pipeline are pictured in Lubmin, northern Germany, on Feb. 15, 2022. Russia clashed with the United States and other Western nations Tuesday, Feb. 21, 2023 over the Kremlin’s call for a U.N. investigation of last September’s sabotage of the Nord Stream 1 and 2 gas pipelines from Russia to Western Europe. (AP Photo/Michael Sohn, File)
WHAT HAPPENED TO THE PIPELINES?
First, Gazprom halted gas flows through Nord Stream 1 on Sept. 2, 2022, citing issues related to European sanctions imposed against Russia over the war in Ukraine.
Three weeks later, both Nord Stream 1 and Nord Stream 2 were hit by explosions that rendered them inoperable and caused significant leaks of gas that was idle in the pipelines. Some have said the blasts caused the worst release of methane in history, although the full extent of the environmental damage remains unclear.
The depth of the pipeline and the complexity of using underwater explosives lent credence to the idea that only a state actor with the expertise to handle such an operation could be responsible. But no one claimed responsibility.
In the immediate aftermath of the explosions, U.S. officials suggested Russia may have been to blame while Russia accused the United States and Britain of being behind them. Investigations by European nations, including Denmark, through whose waters the pipeline travels, and Germany have yet to yield conclusive results.
WHAT THEORIES HAVE BEEN REPORTED?
After months of few developments in the probes, American investigative journalist Seymour Hersh, known for past exposes of U.S. government malfeasance, self-published a lengthy report in February alleging that President Joe Biden had ordered the sabotage, which Hersh said was carried out by the CIA with Norwegian assistance.
That report, based on a single, unidentified source, has been flatly denied by the White House, the CIA and the State Department, and no other news organization has been able to corroborate it. Russia, followed by China, however, leaped on Hersh's reporting, saying it was grounds for a new and impartial investigation conducted by the United Nations.
On Tuesday, though, The New York Times, The Washington Post and German media published stories citing U.S. and other officials as saying there was evidence Ukraine, or at least Ukrainians, may have been responsible. The Ukrainian government has denied involvement.
Germany's Die Zeit newspaper and German public broadcasters ARD and SWR reported that investigators believed that five men and a woman used a yacht hired by a Ukrainian-owned company in Poland to carry out the attack. German federal prosecutors confirmed that a boat was searched in January but have not confirmed the reported findings.
WHAT ARE THE CONSEQUENCES FOR THOSE FOUND RESPONSIBLE?
The implications of a determination that Ukraine was behind the explosions are not entirely clear. It's unlikely it would result in an immediate loss of Western support for Ukraine in the war with Russia, but it might dampen enthusiasm for future assistance if it was found that Ukraine or its agents carried out such an operation in European waters.
A determination that the United States or a proxy was responsible would give Russia and China additional leverage to go after the U.S. and its allies as hypocrites in their demands for the rule of the law, sovereignty and territorial integrity to be respected.
A finding that Russia was behind the explosions would lend weight to Western claims that Moscow is in flagrant breach of international law and willing to use energy as a weapon against Europe.
There is no indication of when the European investigations will be complete — and it seems improbable, given the animosity and mistrust surrounding the Ukraine conflict, that its findings will be universally accepted.
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Photos: In Ukraine, searing images capture a year of war

Natali Sevriukova is overcome with emotion as she stands outside her destroyed apartment building following a rocket attack in Kyiv, Ukraine, Friday, Feb. 25, 2022. (AP Photo/Emilio Morenatti)

Ukrainians crowd under a destroyed bridge as they try to flee by crossing the Irpin River on the outskirts of Kyiv, Ukraine, Saturday, March 5, 2022. (AP Photo/Emilio Morenatti)

Stanislav says goodbye to his 2-year-old son, David, and wife, Anna, after they boarded a train that will take them to Lviv, from the station in Kyiv, Ukraine, Thursday, March 3. 2022. Stanislav stayed to fight as his family sought refuge in a neighboring country. (AP Photo/Emilio Morenatti)

A child in a stroller is lifted across an improvised path as people flee Irpin, on the outskirts of Kyiv, Ukraine, Wednesday, March 9, 2022. (AP Photo/Felipe Dana)

Ukrainian emergency personnel and police officers evacuate injured pregnant woman Iryna Kalinina, 32, from a maternity hospital that was damaged by a Russian airstrike in Mariupol, Ukraine, March 9, 2022. "Kill me now!" she screamed, as they struggled to save her life at another hospital even closer to the frontline. The baby was born dead, and a half-hour later, Iryna died too. (AP Photo/Evgeniy Maloletka)

A man runs after recovering items from a burning shop following a Russian attack in Kharkiv, Ukraine, Friday, March 25, 2022. (AP Photo/Felipe Dana)

An elderly woman is assisted while crossing the Irpin River on an improvised path under a bridge that was destroyed by Ukrainian troops designed to slow any Russian military advance, while fleeing the town of Irpin, Ukraine, March 5, 2022. (AP Photo/Vadim Ghirda)

Bodies are placed into a mass grave on the outskirts of Mariupol, Ukraine, Wednesday, March 9, 2022. (AP Photo/Evgeniy Maloletka)

A woman reacts as she waits for a train trying to leave Kyiv, Ukraine, Thursday, Feb. 24, 2022. (AP Photo/Emilio Morenatti)

Cadets practice with gas masks during a lesson in a bomb shelter on the first day of school at a cadet lyceum in Kyiv, Ukraine, on Sept. 1, 2022. (AP Photo/Efrem Lukatsky)

Armored vehicles destroyed during the fighting between Ukrainian and Russian armed forces lie on a bank of the frozen Siverskiy Donets River in the recently-liberated village of Bogorodychne, Ukraine, Friday, Jan. 13, 2023. (AP Photo/Evgeniy Maloletka)

An explosion erupts from an apartment building at 110 Mytropolytska St., after a Russian army tank fired on it in Mariupol, Ukraine, Friday, March 11, 2022. On the seventh floor of the building, two elderly women Lydya and Nataliya were stuck in their apartment because they couldn't make it down to the shelter, and were killed in the explosion. The two heavily burned bodies were buried by neighbors in front of the building. (AP Photo/Evgeniy Maloletka)

Destroyed Russian tanks sit on a main road after battles near Brovary, north of Kyiv, Ukraine, Thursday, March 10, 2022. (AP Photo/Felipe Dana)

Ira Gavriluk holds her cat as she stands near the bodies of her husband and brother who were killed in Bucha, on the outskirts of Kyiv, Ukraine, Monday, April 4, 2022. (AP Photo/Rodrigo Abd)

Ludmila, left, says goodbye to her granddaughter, Kristina, who, with her son, Yaric, departs by train from Odesa, southern Ukraine, on Tuesday, March 22, 2022. (AP Photo/Petros Giannakouris)

The body of a man with his hands tied behind his back lies on the ground in Bucha, Ukraine, Sunday, April 3, 2022. (AP Photo/Vadim Ghirda)

A man and child ride a bicycle as bodies of civilians lie in the street in the formerly Russian-occupied Kyiv suburb of Bucha, Ukraine, Saturday, April 2, 2022. (AP Photo/Vadim Ghirda)

Children look out of the window of an unheated Lviv-bound train, in Kyiv, Ukraine, Thursday, March 3, 2022. (AP Photo/Vadim Ghirda)

A civilian wears a Vladimir Putin mask as a spoof, while a Ukrainian soldier stands atop a destroyed Russian tank in Bucha, Ukraine, outside of Kyiv, on April 7, 2022. (AP Photo/Rodrigo Abd)

Nina Shevchenko mourns over the body of her 15-year-old son, Artem Shevchenko, who was killed in a Russian attack on Kharkiv, Ukraine, Friday, April 15, 2022. (AP Photo/Felipe Dana)

The body of an elderly woman lies inside a house in Bucha, outskirts of Kyiv, Ukraine, Tuesday, April 5, 2022. (AP Photo/Felipe Dana)

Elderly men lie in beds at a hospice in Chasiv Yar city, Donetsk district, Ukraine, Monday, April 18, 2022. At least 35 men and women, some in wheelchairs and most of them with mobility issues, were helped by volunteers to flee from the region that has been under attack in the last few weeks. They are being transported to Khmelnytskyi, in western Ukraine. (AP Photo/Petros Giannakouris)

Ukrainian soldiers carry the coffin of Volodymyr Losev, 38, during his funeral in Zorya Truda in the Odesa region of Ukraine, Monday, May 16, 2022. The 38-year-old Ukrainian volunteer soldier was killed on May 7 when the military vehicle he was driving ran over a mine in eastern Ukraine. (AP Photo/Francisco Seco)

Nila Zelinska holds her granddaughter's doll found in her destroyed house in Potashnya on the outskirts of Kyiv, Ukraine, Tuesday, May 31, 2022. Zelinska had just returned to her hometown after escaping war to find out she is homeless. (AP Photo/Natacha Pisarenko)

Volodymyr, 66, injured from a strike, sits on a chair in his damaged apartment in Kramatorsk, Donetsk region, eastern Ukraine, Thursday, July 7, 2022. (AP Photo/Nariman El-Mofty)

Relatives and friends pay their last respects to Liza, a 4-year-old girl killed in a Russian attack, during a mourning ceremony in an Orthodox church in Vinnytsia, Ukraine, Sunday, July 17, 2022. Wearing a blue denim jacket with flowers, Liza was among 23 people killed, including two boys aged 7 and 8, in a missile strike three days earlier in Vinnytsia. Her mother, Iryna Dmytrieva, was among the scores injured. (AP Photo/Efrem Lukatsky)

Anastasia Ohrimenko, 26, is comforted by relatives as she cries next to the coffin of her husband, Yury Styglyuk, a Ukrainian serviceman who died in combat on Aug. 24, in Maryinka, Donetsk, during his funeral in Bucha, Ukraine, Wednesday, Aug. 31, 2022. (AP Photo/Emilio Morenatti)

Ukrainian soldiers fire artillery towards Russian positions near Bakhmut, Donetsk region, Ukraine, Sunday, Nov. 20, 2022. (AP Photo/LIBKOS)

A woman warms her dog in her coat in Kivsharivka, Ukraine, Sunday, Oct. 16, 2022. Residents in Kivsharivka have been living without gas, electricity or running water for around three weeks. (AP Photo/Francisco Seco)

Ukrainian family members reunite for the first time since Russian troops withdrew from the Kherson region in the village of Tsentralne, southern Ukraine, on Nov. 13, 2022. (AP Photo/Bernat Armangue)

A resident wounded after a Russian attack lies inside an ambulance before being taken to a hospital in Kherson, southern Ukraine, Thursday, Nov. 24, 2022. (AP Photo/Bernat Armangue)

A woman transporting the coffin holding the body of her son, a soldier who was killed in fighting with Russians, sits in a boat crossing the Siverskyi Donets River near Staryi Saltiv, Kharkiv region on Wednesday Jan. 4, 2023. (AP Photo/Erik Marmor)

A woman walks with a flashlight during a power outage in Kyiv, Ukraine, Wednesday, Dec. 14, 2022. (AP Photo/Felipe Dana)

Ukrainian military doctors treat an injured comrade who was evacuated from the battlefield at the hospital in Donetsk region, Ukraine, Monday, Jan. 9, 2023. The serviceman did not survive. (AP Photo/Evgeniy Maloletka)

The body of a woman lies under rubble after a Russian rocket hit a multistory building leaving many people under debris in the southeastern city of Dnipro, Ukraine, Saturday, Jan. 14, 2023. (AP Photo/Evgeniy Maloletka)

Emergency workers clear the rubble after a Russian rocket hit a multistory building leaving many people under debris in the southeastern city of Dnipro, Ukraine, Saturday, Jan. 14, 2023. (AP Photo/Evgeniy Maloletka)

Relatives mourn over the body of Oleksiy Zavadskyi, a Ukrainian serviceman who died in combat on Jan. 15 in Bakhmut, during his funeral in Bucha, Ukraine, on Thursday, Jan. 19, 2023. (AP Photo/Daniel Cole)

Ambulance paramedic Oleksandr Konovalov performs CPR on a girl injured by shelling in a residential area, next to her father, left, after arriving at the city hospital in Mariupol, eastern Ukraine, Sunday, Feb. 27, 2022. The girl did not survive. (AP Photo/Evgeniy Maloletka)

Residents prepare tea in a basement being used as a bomb shelter in Irpin, on the outskirts of Kyiv, Ukraine, Sunday, March 13, 2022. (AP Photo/Felipe Dana)

A woman takes shelter in a basement with no electricity in Irpin, on the outskirts of Kyiv, Ukraine, Sunday, March 13, 2022. (AP Photo/Felipe Dana)