Los Angeles ? A blackout hit downtown government buildings, Chinatown and adjacent areas Tuesday, but backup power kept key parts of City Hall and police headquarters running. It was the third significant electrical failure in the city since mid-September.

The blackout began about 9 a.m. and cut power to as many as 1,000 customers, affecting City Hall, the Los Angeles County Hall of Administration and police headquarters at Parker Center, said Gale Harris, a spokeswoman for the city Department of Water and Power, which provides electricity to 1.4 million customers.

Power was restored shortly before 6 p.m. The cause of the outage remained under investigation, although officials ruled out terrorism or human error, said Carol Tucker, another DWP spokeswoman. She said brief power outages were likely through the night, as engineers planned to test the system.

“Outages just happen periodically, but we do seem to be having an inordinate number,” Tucker said.

About 100 downtown commercial customers were still without power at midafternoon.

Backup power kicked in at police headquarters, though parts of the complex were dark until electricity was fully restored there before noon, Harris said. A City Council meeting was in progress when the lights went out, but the meeting continued as City Hall switched to backup power.