The UN envoy to Libya has called for an investigation into the murders of 12 men soon after their release from jail in the capital Tripoli last week.
They had been accused of taking part in the killings and torture of anti-government protesters in 2011, under then-leader Col Muammar Gaddafi.
The bodies of the 12 men were found in various parts of Tripoli last Friday, a day after their release.
All had been beaten and shot in the chest and head, their families say.
On Monday Martin Kobler, Special Representative of the UN Support Mission in Libya said he was “utterly shocked” by this “vile crime”.
He called for the killings to “be thoroughly and independently investigated”.
“I urge the relevant Libyan authorities to establish a joint national – international investigation and I will follow developments closely,” Mr Kobler added.
BBC North Africa correspondent Rana Jawad says that those killed had been granted conditional release.
The exact circumstances of the killings are unclear. Officials say the men left al-Baraka prison with their families on Thursday but this could not be independently verified.
Most prisons in Libya are controlled by a combined force of militia groups and the judicial police.
Libya still has rival regional administrations and armed forces in the east and west of the country, with each battling extremist militants in their territories.
The internationally-backed unity government in Tripoli has condemned the killings. It is also carrying out a big offensive against so-called Islamic State in the port city of Sirte.
In a statement, the rival government based in eastern Libya accused the prison authorities of carrying out the executions and dumping their bodies.
They described them as “outlawed groups that control the jail”.
There are hundreds on inmates in al-Baraka, many of whom are accused of being Gaddafi loyalists imprisoned after he was overthrown in 2011.
Last year inmates complained that prison guards regularly beat them and administered electric shocks, Human Rights Watch reported.
Thousands of people including children are arbitrarily detained in Libya, the UN says.