Footage seen by US senators shows two unarmed, shirtless men struggling to stay afloat before they were killed, sources say

• Experts say Trump’s ‘drug boat’ war is legally shaky

Two men who survived a US airstrike on a suspected drug smuggling boat in the Caribbean clung to the wreckage for an hour before they were killed in a second attack, according to a video of the episode shown to senators in Washington.

The men were shirtless, unarmed and carried no visible radio or other communications equipment. They also appeared to have no idea what had just hit them, or that the US military was weighing whether to finish them off, two sources familiar with the recording told Reuters.

Democrat Jim Himes calls footage ‘one of the most troubling scenes’ he’s observed in public service

• Analysis: Killing of survivors sparks outrage – but entire US ‘drug boat’ war is legally shaky

Top Democratic and Republican lawmakers in Congress on Thursday said that the defense secretary, Pete Hegseth, had not ordered the military to kill surviving members of a deadly attack on a boat alleged to be carrying drugs in the Caribbean, but differed over whether the double strike was appropriate.

The allegation that Hegseth ordered the killing of survivors sparked bipartisan concern in Washington that he or others involved may have committed a war crime. On Thursday, US navy admiral Frank Bradley, who commanded the attack, and the chairman of the joint chiefs of staff, Dan Caine, appeared before the House and Senate’s armed services and intelligence committees for a closed briefing in which they showed video and discussed the attack with lawmakers.