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IMPERIAL BEACH, Calif. -- Imperial Beach residents are concerned about a stench lingering from a 143 million-gallon sewage spill into the Tijuana River.

"This is the worst spill we've had in over a decade," Imperial Beach Mayor Serge Dedina told FOX 5.

The Border Environment Cooperation Commission has identified what they believe was the cause -- and it wasn't storm-runoff.

"More than 143 million gallons of raw sewage were discharged into the Tijuana River, which flowed through south San Diego, into Imperial Beach, closed beaches from the border all the way to Coronado," Dedina said.

"This is the worst spill we've had in over a decade."

Sewage spill report from #IBWC--143 million gallons spilled into TJ Rver starting on 2/6/17. This is why @WILDCOAST @SurfriderSD are needed. pic.twitter.com/P4gySzlxEu — Serge Dedina (@sergededina) February 25, 2017

Dedina says sewage accidents happen from time to time, but in this case, he's not sure it was a complete accident.

"If it would've been a sewage spill or a malfunction maybe they would've asked for help, but because it went on for so long, it was so much, possibly they were fixing something and decided just to not to let us know," Dedina said.

The report, issued by the National Pollutant Discharge Elimination System, noted the spill flow started on February 6 and lasted for 17 days. It sites "rehabilitation of major sewer collector" as the probable cause of the spill.

"We asked authorities to let us know what was going on and we just heard no response," Dedina said.

Dedina says the cross-border partners weren't acting as partners.

FOX 5 reached out to the Mexican consulate but did not receive a response.

Dedina says he doesn't think the spill was a breakdown in Mexico-U.S. relations but says this environmental crisis must be avoided in the future.

"There is a difference between rain-related runoff, which we all get in San Diego, and the deliberate spilling of sewage for 17 days," Dedina said.

The city's beaches remained closed Friday evening as lifeguards continued to test the water. Lifeguards told FOX 5 they don't know when water conditions will return to a safe level. Beachgoers were advised to make beach plans north of the area.