U.N. investigators accused a Saudi Arabia-led military coalition of carrying out a deadly March attack on a Somali migrant boat off Yemen and said the alliance had become a cover for some states to avoid individual blame, according to a confidential report seen by Reuters on Wednesday.

The coalition fighting the Iran-allied Houthi militia in Yemen has denied striking the boat in the Red Sea near the port of Hodeidah. The investigators said the attack killed 42 people and injured 34 of the more than 140 people onboard.

“This civilian vessel was almost certainly attacked using a 7.62 mm caliber weapon from an armed utility helicopter,” the investigators, who monitor sanctions in Yemen, wrote in a 185-page report to the U.N. Security Council on Monday.

“The Saudi Arabia led coalition forces are the only parties to the conflict that have the capability to operate armed utility helicopters in the area,” the report said. It said the helicopter was likely operating from a naval vessel.