Authorities in the Russian city of Izhevsk declared a state of emergency on Friday due to an oil spill on the Starkovka River that took place nearly a month ago.

The cause of the May 29 fuel leak, which encompasses an area of 640 square meters in the city near the Volga River, has not yet been determined.

Specialists from the regional Natural Resources and Environment Ministry and the Izhevsk civil protection administration collected water samples at three points along the river.

Ministry data cited by the Kommersant business daily reported that there were “elevated concentrations of oil products” in the water.

Boom barriers were installed near the dam where the oil spill was found, authorities said, and additional barriers are planned upstream to help determine where the oil entered the Starkovka.

The Natural Resources Ministry said it did not identify damage to “water resources” and that there was no threat of the spill spreading beyond the initial site of the leak.

“The cause of the river’s contamination must be established within the next two weeks, and comprehensive measures must be taken to address the effects of the spill,” the Izhevsk city administration said.

Officials promised to clean up oil products from the river’s surface and to inspect and clean both the riverbed and the shoreline.

The emergency regime will stay in place until further notice, Izhevsk Mayor Dmitry Chistyakov said.

Law enforcement agencies have opened a probe into the incident.