Federal officials are investigating why millions of gallons of sewage-laden water isn’t making its way from Tijuana to the international wastewater treatment plant in the U.S. Instead, that untreated wastewater is flowing into San Diego through a border drain, which indicates there’s probably a broken pipe or a clog somewhere in Tijuana.

The runaway flow began Jan. 7 around 1:30 p.m. when almost a million gallons of sewage escaped from Tijuana through Stewart’s Drain, which sits just east of the International Wastewater Treatment plant operated by the International Boundary Water Commission.