The Minnesota Pollution Control Agency (MPCA) was responding Tuesday to a possible contaminant spill in the Mississippi River in the Twin Cities area.

The agency said a sheen on the water's surface was spotted on the river at 1 p.m. Tuesday. It has been blocked from entering drinking water sources for Minneapolis and St. Paul and Xcel Energy's Riverside plant.

"There is no immediate drinking water threat or public health threat," Fridley Fire Chief Maddison Zikmund said.

Andrea Cournoyer, a spokesperson for the MPCA, said it is not yet confirmed what substance was making the sheen or its total volume. An analysis of water samples is pending.

Zikmund said preliminary test results suggest the substance is not gasoline or a similar product.

"We're at a loss here," he said.

Racquel Vaske, the general manager for St. Paul Regional Water Services, the water utility serving the city and suburbs, said the origin of the sheen is also unclear. But she emphasized that residents do not have to fear contamination in drinking water.

"It is not a concern for our customers," she said. "There is no impact to them at this point, nor will there be."

The sheen, first reported by the Fridley Fire Department, was seen from the Hwy. 610 bridge in Coon Rapids to a location in Fridley about 2 miles downstream, Cournoyer said. It has since begun dissipating.

No other information was immediately available late Tuesday.