United Nations investigators have found “convincing evidence” that sarin gas was used in a chemical weapons attack near Damascus last month.

Investigator Ake Sellstrom handed the report to UN Secretary-General Ban Ki-Moon. While the report has yet to be officially released, a photograph of the transfer of the document showed its first page.

The text that could be seen noted that “environmental, chemical and medical samples” collected at the scene of the attack “provide clear and convincing evidence that surface-to-surface rockets containing the nerve gas sarin were used” in the assault in the Ghouta area.

The report appears not to have apportioned blame for the attack, as UN investigators warned that it would not.

“The conclusion is that chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic,” the report stated – but the question of who used the chemical weapons remains open.

United States leaders have accused Syrian President Bashar Assad of being behind the massacre. Assad has accused Syrian rebel groups of perpetrating the attack.

A UN commission tasked with investigating human rights abuses in Syria reported last week that the Assad regime was behind eight massacres. Both sides have committed war crimes, the commission noted, including mass executions, rape and torture.