Authorities in Russia are investigating a Nigerian criminal network that allegedly forced women into prostitution after they arrived visa-free during Russia’s football World Cup in 2018, the state-run TASS news agency reported 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, Reuters reported in March.

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 TASS.

“[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 deny their guilt, according to TASS.