The U.N. envoy to Libya called on Monday for investigations inside and outside Libya into the killing of 12 men jailed on suspicion of crimes against protesters during the 2011 revolution and granted conditional release last week.

Libya's judicial police say the men were released from Tripoli's Al-Baraka prison on Thursday, a day before their bodies were found dumped in different parts of the capital.

According to their families, the men had been beaten savagely, shot in the chest and head, and some had their hands and feet tied.

"This crime should be thoroughly and independently investigated and perpetrators must face justice," said U.N. envoy Martin Kobler. "I urge the relevant Libyan authorities to establish a joint national-international investigation and I will follow developments closely."

The attorney general in Tripoli has said the case will be investigated. The U.N.-backed Government of National Accord (GNA) in Tripoli has also called for an urgent investigation.

The United Nations said the killings may constitute international crimes under the jurisdiction of the International Criminal Court.

The case is sensitive because the GNA arrived in March with the acquiescence of some of the armed factions that have long controlled Tripoli and acted with impunity there. Many were originally formed of rebels who took part in the 2011 uprising that toppled Muammar Gaddafi, and present themselves as guardians of the revolution.

The GNA, which is designed to replace two rival governments set up in 2014 in Tripoli and the east, says it is working to integrate armed groups in Tripoli into a national security force as it tries to ease the previous government in the capital aside.

But its critics, especially in the east, have accused the GNA of legitimising militias operating in western Libya by seeking their cooperation and not acting quickly enough to disband them.

The eastern government, parliament and military forces all released statements on the prison killings, with the government pointing the finger at "outlawed groups that control the jail".

Al-Baraka prison houses hundreds of inmates, many of whom are accused of being Gaddafi loyalists.

Last year detainees at the jail told Human Rights Watch that prison guards frequently beat them, and administered electric shocks.

Thousands of people including women and children are detained in Libya, many of them arbitrarily, the United Nations says.