Seventeen million gallons of raw sewage were discharged into Santa Monica Bay earlier this week after a sprawling wastewater treatment plant in Los Angeles experienced problems, prompting several beach closures more than 12 hours later that were sharply criticized for not happening sooner.

The beaches, which stretch for more than 2.5 miles along the Pacific Coast west of Los Angeles International Airport, reopened on Wednesday after a two-day closure, according to public health officials.

But the fallout from the handling of the sewage release from the Hyperion Water Reclamation Plant, one of the largest wastewater treatment plants in the world, continued well after swimmers and surfers were allowed to return to the ocean.

Critics said that the city’s Department of Public Works and the Los Angeles County Department of Public Health took more than 12 hours to alert beachgoers about the discharge, which lasted about eight hours — from Sunday night into Monday morning.

It wasn’t until late on Monday morning that the first beach closure signs appeared along the nearby stretch of coastline, The Los Angeles Times reported. The county’s health agency did not mention the closures until 5:30 p.m. local time on Monday, when it posted a news release on its Twitter account.

On Tuesday, the Los Angeles County Board of Supervisors called for an inquiry into what went wrong at the sewage treatment plant and the response of the city and county.

“What happened yesterday off the coast of Dockweiler Beach was irresponsible, unacceptable and dangerous,” County Supervisor Janice Hahn said Tuesday on Twitter. “Not only did the Hyperion Plant release 17 million gallons of sewage into our ocean, the public had little to no information about it for HOURS.”

The city’s public works agency referred on Thursday to a previous statement provided by the plant’s executive manager, saying that the plant’s operators had followed the protocols for notifying local and state agencies about the emergency discharge.

The executive manager, Timeyin Dafeta, said that the plant had become inundated with overwhelming quantities of debris on Sunday afternoon, causing backup of the headworks facilities.

“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,” Mr. Dafeta said.

The 17 million gallons of raw sewage that were released represented about 6 percent of the average daily load at the facility, according to the city’s estimates.

“We are investigating the cause of the debris (primarily construction and landscaping materials, and grease balls), and are repairing damaged equipment,” Mr. Dafeta said.

The Los Angeles County Health Department did not immediately comment on Thursday when asked about why it took about half a day for beach closure signs to be posted.

In a news release on Wednesday, the agency said that water samples that it had taken during the past two days from the four beaches that were closed were acceptable for reopening them. These beaches were: Dockweiler State Beach at Water Way Extension, Dockweiler State Beach at Hyperion Plant, El Segundo Beach and the beach near the Grand Avenue storm drain.

Untreated sewage and polluted water that runs off after it rains can expose swimmers to pathogens, according to the U.S. Environmental Protection Agency, which said in a post on its website that children, older people and those with compromised immune systems are at greater risk of illness.

The most common illness associated with exposure to sewage is gastroenteritis, which the E.P.A. said can cause nausea, vomiting, stomachache, diarrhea, headache or fever. Ear, eye, nose and throat infections are also a risk.

In a motion that was unanimously approved on Tuesday, the county’s Board of Supervisors also directed officials to develop an action plan so that they could respond to future sewage discharges in a more timely manner.