SAN ONOFRE (CNS) - Officials say they don't know what caused a computer used to monitor security on the grounds of the San Onofre nuclear generating plant to shut down, prompting the issuance of an "unusual event" report -- the lowest emergency level.

The computer, which monitors gates and doors, went down around 6:10 a.m. Saturday and was restarted 45 minutes later without incident, officials said, adding that security at the plant was not compromised.

Although the computer problem is regarded as minor, officials said the event would be investigated and reported to the Nuclear Energy Commission consistent with reporting requirements pertaining to nuclear plants.