(CNN) -- With millions of gallons of raw sewage spilled into the Pacific Ocean surface and onto the shores of southern California, officials in the United States and Mexico have launched a bi-national investigation into the spill that apparently originated in Mexico.
The spill was first spotted in early February along the beaches of San Diego, California down through Tijuana, Mexico, according to the International Boundary and Water Commission. The IBWC is a joint government agency responsible for brokering water treaties between the two countries.
Agency commissioners agreed to investigate the matter after receiving reports about sewage originating in Mexico that affected the Tijuana River, Tijuana River Estuary, and coastal waters at Imperial Beach, California.
"This is the worst spill we've had in over a decade," Imperial Beach Mayor Serge Dedina told CNN affiliate KSWB. "More than 143 million gallons of raw sewage were discharged into the Tijuana River, which flowed through south San Diego, into Imperial Beach (causing) closed beaches from the border all the way to Coronado."
Coronado beaches opened for the first time in several weeks Sunday, but stretches of beach further south from Silver Strand State Beach to Imperial Beach remain off limits, KSWB reports.
"The ocean shoreline from the International Border to the north end of Silver Strand at Avenida Lunar will remain closed until sampling confirms these areas are safe for water contact," the San Diego County Department of Environmental Health said in a statement.
Initial reports from Imperial Beach city officials indicate the sewage came from a broken sewer pipe in Tijuana, KSWB reports.
"We need to make sure the commission receives timely and accurate information when there are sewage spills in one country that affect the other," IBWC Commissioner Edward Drusina said in a release.
The duration of the spill and volume have not been confirmed, IBWC officials said. The investigation is expected to last 30 days and results are expected by April 1.
Marine conservationist are also expressing concern over the border sewage.
For now, yellow and red signs lining the shores of Imperial Beach warn beach goers away from the coastal waters.
CNN's Joe Sutton contributed to this report.
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