San Francisco - Parts of America's radiation alert network have been out of order during Japan's nuclear crisis, raising concerns among some lawmakers about whether the system could safeguard the country in a future disaster.

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.