BROWNSVILLE, Texas (AP) _ A toxic chemical that spilled into San Martin Lake last month may be to blame for the deaths of 6 million fish and dozens of ducks, state officials said.

On Jan. 25, a tanker-truck carrying 9,000 gallons of furfural overturned. Some of the toxic chemical, used in manufacturing, spilled into a drainage ditch that flows into San Martin Lake.

Officials dammed up the ditch at three places. Last week, the Coast Guard released water from two temporary dams; a third dam was opened but then rebuilt when the dead fish were found.

Coast Guard officials who released the water from the spill said it was tested beforehand and found to have safe levels of furfural.

State officials said they were trying to figure out why so many fish died.

``We are looking for it (furfural) everywhere and any way we can. But in addition to that, we are looking for any other contaminant that may be in that ditch,'' said Charlie Webster of the Texas Natural Resource Conservation Commission.

San Martin Lake is located between Brownsville and the Gulf of Mexico.

Investigators also were performing tests to check whether 40 to 50 ducks found dead or dying on nearby South Padre Island were related to the spill. Officials feared the birds may have been feeding off the dead fish.

Meanwhile, an arrest warrant was issued for Santos Mejia Banda, of Mexico, the tanker-truck's driver. Police said Banda presented them false insurance papers and a fake Mexican driver's license. Banda's lawyer said the documents were legitimate.