Police Thursday found part of a rocket-propelled grenade launcher in a park near the scene of an attack on the headquarters of Britain's MI6 intelligence service.
It was the first time the Russian-built Mark 22 anti-tank weapon has been used in England, said Deputy Assistant Commissioner Alan Fry, head of the police anti-terrorist branch.
In February, a similar weapon was found in Dungannon, Northern Ireland, and was believed to have been hidden by the Real IRA, an Irish Republican Army dissident group opposed to the 1998 peace agreement.
"It's designed to go through tanks, and therefore it is fairly impressive against metal," Fry said, adding that it could go through about one yard of concrete.
He said the attacker probably had some military training, although "the skill necessary to use this weapon takes about 30 minutes to acquire."
He said the Real IRA was the leading suspect, but he did not rule out other groups.
No one has claimed responsibility for the Wednesday night attack, which came without warning but injured no one.
Damage was limited to a window and two wall panels on the eighth floor.