Omsk, in southwestern Siberia, was coated twice last winter with a blanket of black snow after a thermal power station dubbed TPP-5 kicked into overdrive.
"According to preliminary reports, this is again TPP-5 that has dumped ash," the watchdog group was quoted as saying Sunday by state news agency RIA Novosti.
The report said $4.3 million was slated for new filters at the facility.
A team of researchers from the environmental agency is expected in Omsk this week to collect samples. There was no indication if the black material was a threat to the city's population.
A white powder that later was discovered to be out to be aluminosilicate dust from a Gazprom Neft oil refinery coated Omsk in August 2012.