Families of victims of the deadliest US air disaster since 2001 visited the crash site on Sunday just outside Washington, walking along the banks of the Potomac River near Reagan National Airport to memorialise their loved ones.

Dozens of people arrived in buses with a police escort close to where an American Airlines jet and a US Army Black Hawk helicopter collided on Wednesday, killing all 67 aboard the two aircraft. Federal investigators were working to piece together the events that led to the crash while recovery crews were set to pull more wreckage from the chilly water.

Transport Secretary Sean Duffy on Sunday said he wanted to leave federal aviation investigators space to conduct their inquiry.

But he posed a range of questions about the crash while appearing on morning television news programmes.

“What was happening inside the towers? Were they understaffed? … The position of the Black Hawk, the elevation of the Black Hawk, were the pilots of the Black Hawk wearing night vision goggles?” Duffy asked on CNN.