Part of the nation's key radiation warning system was out of service as the U.S. braced for possible exposure to the fallout from a nuclear crisis in Japan.
Also, 20 monitors out of 124 nationwide were out of service this week, including units in Buffalo, N.Y., and Harlingen, Texas, according to the Environmental Protection Agency.
While no dangerous levels of radiation have reached American shores, the test of the monitoring network has spurred some lawmakers to question whether it can adequately safeguard the country.
The system is crucial because federal officials use the monitors' readings to validate the impact of nuclear incidents, then alert local governments and the public.
In California, federal officials said four of the 11 stationary monitors were offline for repairs or maintenance last week.
The EPA said the machines operate outdoors year-round and periodically need maintenance, but some were not fixed until a few days after low levels of radiation began drifting toward the mainland U.S.
Gaps in the system and delays in fixing monitors in some of Southern California's most populated areas have prompted hearings and inquiries in Washington and Sacramento.
"Because the monitoring system plays such a critical role in protecting the health and safety of the American people, we will examine how well our current monitoring system has performed in the aftermath of the tragic situation in Japan," said Sen. Barbara Boxer, D-Calif., who heads the U.S. Senate Environment and Public Works Committee, which plans a hearing in the coming weeks.
EPA officials said the program effectively safeguarded the country against a threat that did not materialize. They said they put portable monitors in place as backups and repaired the permanent ones in Los Angeles, San Bernardino and San Diego last weekend.
The EPA's independent watchdog, Inspector General Arthur Elkins, said he is considering reviewing the agency's emergency response planning, including its RadNet system.
The network, launched after the Cold War and upgraded following the 9/1 1 attacks, measures radiation nationwide through dozens of monitors that suck in air samples periodically and pump out readings about radioactive isotopes.
The EPA's data is sent to the Department of Energy's National Atmospheric Release Advisory Center in California. Teams there check it against sophisticated computer models that predict how releases at the Fukushima plant in Japan could spread across the Pacific.
To save money, EPA relies in part on trained volunteers to regularly change out air filters on the RadNet monitors and mail them to a federal lab in Alabama for analysis. Volunteers also alert EPA if something goes wrong with the machine.