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The release of about 17 million gallons of untreated sewage from the city of Los Angeles’ largest treatment plant closed miles of beaches to swimming from the LAX area to El Segundo Monday, officials said.

A power outage Sunday night caused sewage from the Hyperion Water Reclamation Plant in Playa del Rey to spill into the ocean, L.A. County Supervisor Janice Hahn said in a tweet.

“Water samples are being tested and I’m getting more information about the scope of the problem,” Hahn said.

The public was urged to avoid going into the ocean in affected areas, which the county Department of Public Health listed as Dockweiler State Beach, El Segundo Beach and the Grand Avenue storm drain.

Closure signs were posted in the area Monday afternoon. They will remain closed until water samples are confirmed negative for elevated bacteria, with the first testing results expected in 24 hours, public health officials said.

Officials say the unfiltered sewage was discharged into the Pacific through pipes that extend 1 mile and 5 miles offshore.

The facility “became inundated with overwhelming quantities of debris, causing backup of the headworks facilities,” Hyperion Executive Plant Manager Timeyin Dafeta said in a statement to the Associated Press.

“The plant’s relief system was triggered and sewage flows were controlled through use of the plant’s one-mile outfall and discharge of untreated sewage into Santa Monica Bay,” Dafeta said.

Hanh tweeted that officials are “going to need answers about how and why this happened.”

Although the 17 million gallons of sewage that seeped out is a large amount, it still doesn’t match up to L.A.’s largest spill. That was in 1998, when more than 30 million gallons of sewage spilled during El Niño storms, according to the Los Angeles Times.

The Hyperion plant is not only the city’s largest but also its oldest sewage treatment facility, in operation since 1894. An average of 275 million gallons of wastewater flow into the plant on any given dry-weather day.

Information on current beach conditions is available on the county’s 24-hour hotline at 800-525-5662.

Correction: A previous version of this story incorrectly stated the year the Hyperion plant began operating. This post has been updated.