Authorities in Russia are currently investigating a Nigerian criminal network that allegedly forces women into prostitution after they arrive visa-free during Russia’s football World Cup in 2018, newsmen reported on Monday.

Two Nigerians were arrested in October 2018 and charged with human trafficking after selling one victim to a police officer posing as a client, Russian media reported in December. Activists say that many of the estimated 2,000 Nigerian women who entered Russia on a World Cup Fan ID are still in slavery.

The two suspects in the human trafficking case were allegedly part of a criminal network that forced young Nigerian women into prostitution, an unnamed Russian law enforcement source told newsmen.

“[Both] are witnesses in another criminal case [into organizing prostitution] currently being investigated,” the source was quoted as saying.

The defendants in the organizing prostitution case are also Nigerian nationals, the source said without indicating the overall number of suspects.

The source did not say which stage that case had reached.

The two human trafficking defendants’ next hearing is scheduled for Jan. 22, according to the court website.

Both plead not guilty according to newsmen.