Federal officials say the system of sensors has helped them to validate the impact of nuclear fallout from the overheated Fukushima reactor, and in turn alert local governments and the public. They say no dangerous levels of radiation have reached U.S. shores.

In California, home to two seaside nuclear plants located close to earthquake fault lines, federal authorities said four of the 11 stationary monitors were offline for repairs or maintenance last week. The Environmental Protection Agency said the machines operate outdoors year-round and periodically need maintenance, but did not fix them until a few days after low levels of radiation began drifting toward the mainland U.S.

About 20 monitors out of 124 nationwide were out of service earlier this week, including units in Harlingen, Tex. and Buffalo, N.Y. on Friday, according to the EPA.

Gaps in the system — as well as the delays in fixing monitors in some of Southern California’s most populated areas — have helped to prompt 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, a California Democrat who chairs the U.S. Senate Environment and Public Works Committee, which plans a hearing in the coming weeks on nuclear safety.

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, San Diego last weekend.

"The network as a whole continues to detect even the slightest traces of radiation in the air," the agency said in a statement to The Associated Press.

The EPA’s independent watchdog, Inspector General Arthur Elkins, told the AP he is considering reviewing the agency’s emergency response planning, including the agency’s RadNet system.

The network, launched after the Cold War and upgraded following the 9-11 attacks, measures radiation nationwide through dozens of monitors that suck in air samples periodically and pump out real-time readings about radioactive isotopes.

The EPA’s data, as well as samples that numerous federal agencies are collecting in Japan, 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 Fukushima 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 where the data gets a detailed analysis a few days later. Volunteers are also tasked with alerting EPA if something goes wrong with the machine.

"It sounds sort of loosey goosey, but we already operate our network on a very rigid schedule so we just sort of fit it into our lifestyle," said Eric Stevenson, a director of technical services who oversees operation of the monitor from his office at the Bay Area Air Quality Management District near San Francisco’s domed city hall. "We’ve been operating this thing for years and no one has really said boo about it. Something like this comes along and all of us realize ‘Hey, gee, that’s a relatively smart program.’"

One RadNet monitor in Fontana, Calif. stopped transmitting data in November, and regional air quality officials alerted EPA, said Philip Fine, an atmospheric measurements manager with Southern California Air Quality Management District. The repairs happened last weekend, when EPA made fixing California monitors a priority, he said.

In San Diego, an air district official who oversees one RadNet monitor, said they "babysit" the machine for the EPA and were unaware it had problems until agency officials showed up to fix it last weekend.

"We thought it was running," said Bob Kard, the air pollution control officer for the San Diego Air Pollution Control District.

EPA officials say the system has more than enough monitors to detect any radiation problems even if individual machines break down.

"We have plenty of data coming in across the country to see the potentials on health and safety," said Ron Fraass, who directs EPA’s National Air and Radiation lab in Montgomery, Ala. "If you were going to keep your pc operating outdoors in all weather, it’s going to break once in a while."

California lawmakers have questioned the adequacy of the EPA monitoring, noting there are no sensors along the coast between San Francisco and Los Angeles.

"The question remains unanswered as to why we have gaps," said California Senate Majority Leader Ellen M. Corbett, (D-San Leandro), who chairs the state committee on earthquake and disaster preparedness "The radiation monitors that we do have in California must be properly checked and maintained."

Daniel Hirsch, a nuclear watchdog who lectures on nuclear policy at the University of California, Santa Cruz, said he was uneasy about malfunctioning monitors.

"The fundamental concern is that we’re being offered bland assurances that everything is ok but much of the monitoring system was broken," Hirsch said.