MOSCOW, August 21 (RIA Novosti) – Four Russians have been charged with smuggling timber valued at 100 million rubles ($3 million) a month into neighboring China, police said Wednesday.

Mikhail Burlyaev, Larisa Kuzmina and their wives Sun Dzhendzun and Olesya Mulchak are accused of using dozens of different companies and fake documents to smuggle 150 freight cars of lumber every month, according to a police statement posted online.

Investigators from Irkutsk Region, Novosibirsk Region and Leningrad Region are continuing their investigation into the smuggling ring, police said. “We have received information that the suspects are just one small part of a larger group carrying out this criminal activity,” the police statement said.

The timber trade across the Russia-Chinese border is driven by enormous Chinese demand for raw materials and the potential supply from the vast forests of southern Siberia. Illegal logging increased dramatically after the collapse of the Soviet Union and has had severe ecological consequences for local forests, environmentalists say.

Russia exported 24.2 million cubic meters of logs and sawn wood to China in 2011, according to a 2012 report published by the Environmental Investigation Agency. The illegal trade in the same period was about 10 million cubic meters of logs and sawn timber, valued at $1.3 billion, the report said.

President Vladimir Putin strongly criticized the oversight of Russia’s forestry industry during an April meeting in Ulan Ude, the capital of the Buryatia republic. “Forests must be saved from illegal logging,” he said, Rossiiskaya Gazeta reported. “The industry is in a critical condition.”