Last week, the World Health Organization warned that Russia risks developing an out-of-control HIV epidemic after data showed a record number of new cases last year. Russian prisons have in the past been scrutinized for their treatment of HIV patients, with reports from the Prosecutor General’s Office in 2016 saying that prisoners do not receive the medications needed to treat HIV and hepatitis.

A prominent Russian human rights lawyer reported Tuesday that a massive HIV outbreak had broken at a prison in the Yaroslavl region — a claim that has since been denied by federal prison officials.

Irina Biryukova, a lawyer working for the Public Verdict human rights group, wrote in a post on her Facebook page Tuesday that around 150 prisoners are believed to have contracted HIV over the past month and a half at a penal colony in Yaroslavl, based on reports from family members of the prisoners.

“Right now, information is coming in from various sources, including relatives and friends of convicts from the IK-2 Penal Colony,” Biryukova said.

According to the lawyer, Russia’s Federal Penitentiary Service (FSIN) has launched an investigation into the outbreak and a criminal case has been opened against the former head of the prison.

The press service of the local FSIN branch on Tuesday dismissed Biryukova’s story, telling local media that the reports were “untrue.”

The local penitentiary service also stressed that all prisoners have to pass mandatory tests for HIV upon their admission and release from the colony.

The Public Verdict Foundation was established in February 2004 as a non-profit, non-partisan organization offering legal assistance to victims of human rights abuse at the hands of law enforcement officers in Russia.