Lawmakers, regulators and environmental groups in San Diego County are calling on federal officials to investigate a massive sewage spill in the Tijuana River that some people believe Mexico may have intentionally caused and then refused to explain much about it.

Mexican authorities have yet to give a full accounting of how, without advance notice, an estimated 143 million gallons of effluent spewed into the river during 17 days that ended on Feb. 23. The pollution flowed into South County and has continued to contaminate beaches as far north as Coronado. Signs posted along that entire stretch of coastline urge people to stay out of the water because toxins in the sewage can sicken or kill swimmers.

After water-quality experts, environmentalists and elected officials made inquiries starting in mid-February, the U.S. side of the International Boundary and Water Commission said it received a brief summary from Mexico on Friday saying the discharge resulted from a project to repair a sewer pipe at the confluence of that nation’s Alamar and Tijuana rivers.

Mexican agencies gave no alert of the pipe repair, their U.S. counterparts said. Baja California’s State Public Service Commission maintains the sewer-system infrastructure in the originating area.

That agency and other Mexican officials have not responded to The San Diego Union-Tribune’s requests for comment since Monday.

On Wednesday, Imperial Beach Mayor Serge Dedina, San Diego Councilman David Alvarez, Coronado Mayor Richard Bailey, Rep. Scott Peters, D-San Diego, the clean-water group Wildcoast and the local chapter of the Surfrider Foundation were among those calling on the International Boundary and Water Commission to launch an official investigation into the spill.

As part of their campaign, they’re urging the public to gather at 5:30 p.m. Thursday at 301 Caspian Way in Imperial Beach to coordinate requests for the commission to launch a probe. The panel’s meeting is scheduled to start at 6:30 p.m. in the same location.

The commission’s U.S. side, which oversees international water treaties with Mexico, among other things, hasn’t said whether it will start a formal investigation.

“There’s a deafening silence despite the crescendo of official complaints, and that’s just unacceptable,” said Dedina, who is also the head of Wildcoast.

Alvarez, whose council district includes the Tijuana River Valley, also said the spill shouldn’t be tolerated. “The federal government needs to work with Mexico and … take the necessary steps to address this problem as soon as possible,” he said.

Dave Gibson, executive officer of the San Diego Regional Water Quality Control Board, has said the discharge was likely no mistake. He has pushed for the International Boundary and Water Commission, the U.S. Environmental Protection Agency and other federal regulators to look into the situation.

“I strongly support an investigation, and I am formally calling on (federal officials) to conduct one immediately,” Gibson said. “Whether it is one gallon or 143 million gallons, sewage should never be discharged untreated into our water bodies.

“After working years to improve cooperation and communication on border issues, it is deeply disturbing that (Baja California’s State Public Service Commission) … utterly failed to take any measures to contain or redirect the raw sewage back into the sewage system or provide any public notice whatsoever,” he added.

Peters, whose 52nd congressional district includes Coronado, said: “This is not supposed to happen. I am very concerned about the damage this spill has done to our beaches and bays. As a member of the (House) Energy and Commerce Committee, I plan to speak with the EPA and other relevant federal agencies to get answers on how this happened, why we weren’t informed sooner and what steps need to be taken to ensure it never happens again.”

Rep. Juan Vargas, whose 51st congressional district encompasses much of the coastline affected by the sewage spill, hasn’t said as of Wednesday afternoon whether he will join the call for further scrutiny.

Depending on how much of the sewage is flushed out to sea by recent storms, toxic materials may persist along the region’s beaches into the spring and summer, according to the water quality board. For much of the past month, South County beaches had been closed because of run-off pollution generated by rainfall.

San Diego County’s Department of Environmental Health expanded an existing beach closure on Tuesday because of sewage. The closure now includes the Tijuana Slough National Wildlife Refuge, Imperial Beach the Silver Strand and Coronado.

Officials said warning signs will remain along the shorelines of those sites until the ocean is safe for recreational use.