A deadly missile strike on a migrant detention centre in Libya was carried out by a fighter plane from a foreign country, according to a confidential UN investigation seen by BBC Arabic.
No country is named but a source with knowledge of the inquiry said it was focused on the United Arab Emirates.
The UAE did not comment when approached by the BBC.
July's attack killed 53 migrants and injured 130, and was described as a potential war crime by a UN official.
Most of those killed at the Tajoura migrant detention centre, east of the capital, Tripoli, are believed to have been sub-Saharan Africans attempting to reach Europe from Libya.
It was the highest publicly reported death toll from an attack since the Libyan National Army (LNA) began a fresh offensive in April to topple the UN-backed Government of National Accord (GNA) in Tripoli.
"This attack may, depending on the precise circumstances, amount to a war crime," UN High Commissioner for Human Rights Michelle Bachelet said at the time.
The UN Special Mission in Libya told the BBC it had shared the co-ordinates of migrant centres with both sides in the conflict to prevent them from being hit.
The GNA said in July the attack had been carried out by a fighter plane from the UAE. The LNA, under Gen Khalifa Haftar, initially said it had bombed a legitimate target but later denied being involved.
A panel working for the UN Security Council has spent months trying to establish who was behind the attack.
BBC Arabic saw a confidential report which will be presented to the UN Security Council on Wednesday.
The report cites evidence from a confidential source saying "an unknown number of Mirage 2000-9" fighter jets were operating from two airbases inside Libya at the time of the strike.
The UAE and Egypt, which have both backed the LNA, have a large number of Mirage fighter jets.
A spokesman for the Egyptian Army said it did not want to comment on a report before it had been published.
The UN report says the Mirage jets were using two airbases: Jufra and al-Khadim.
In 2017, the UN said the UAE had built up the air base at al-Khadim and provided air support to Gen Haftar's forces.
The confidential report concludes it is "highly probable" the air strike was carried out using precision-guided missiles by a fighter jet "operated by a [UN] member state acting in direct support of HAF [Haftar Armed Forces]".
The report does not name the state as it says evidence is still being gathered.
The UAE and LNA did not respond to repeated BBC requests for comment.
"If there is concrete evidence of direct military intervention by outside countries, then that is totally unacceptable, and needs to be investigated at the most senior levels," said the former UK ambassador to Libya, Peter Millet.
There has been an arms embargo on Libya since 2011. In August 2019, the UAE signed a commitment to stick to the embargo along with the UK, US, France and Italy.