Air traffic control staffing was “not normal” at the time of a mid-air collision between a military helicopter and passenger plane in Washington, DC that killed 67 people, Transportation Secretary Sean Duffy has acknowledged.

US media reported that Reagan National Airport tower was understaffed during Wednesday’s crash, according to a government report.

One air traffic control worker was managing helicopters and some planes from the airport, a job normally done by two people, two sources told CBS News, the BBC’s US news partner.

“I’ll take the Federal Aviation Administration (FAA) at their word that it wasn’t normal,” Duffy said when asked about reports during the FOX News Sunday programme.

Duffy said “that was part of the review process that we have to do”.

He explained there was a “consolidation of air traffic controllers an hour before it was supposed to happen during the time of this crash. And so was, what was the appropriateness of that?”

Duffy raised questions about whether controllers did “appropriately direct traffic, consistent with procedures at the FAA”, as well as the elevation of the helicopter.

National Transportation Safety Board (NTSB) Board Member Todd Inman said preliminary data revealed the helicopter was flying at about 200ft (60m), CBS reported. The flight ceiling for helicopters in the area near Reagan National Airport is 200ft.

Duffy also questioned the timing of its flight.

“Why would they fly a mission at nine o’clock at night through really busy air space… as opposed to flying that mission at one o’clock in the morning, when there’s very little traffic”? he asked.

“I want our military to be trained up and ready to go, but I also want air travellers to be safe as well, and there’s a time and a place to do it, not at nine o’clock at night when there’s heavy traffic.”

Investigators are considering a number of factors, including height, in determining the cause of the crash, but have not yet made any public conclusions, NTSB officials said on Thursday.

There were 64 passengers aboard the American Airlines flight when it collided mid-air with an Army Black Hawk helicopter carrying three soldiers. There were no survivors.

Dozens of family members of the victims gathered at the scene of the crash on Sunday morning.

They arrived in chartered buses with a police escort, first travelling to the crash scene and then on to a runway where the flight was supposed to land.

Duffy said the US has a chronic understaffing problem in air traffic control.

US media has reported that more than 90% of air traffic control facilities in the country are operating below FAA recommended staffing levels.

“We haven’t had enough air traffic controllers in America for a very long time,” Duffy said, adding “they are stressed out. They’re tapped out. They’re overworked. That’s no excuse. It’s just a reality of what we have in the system.”

The new transportation secretary said he was working with the FAA to train new air traffic controllers and “has a plan that’s going to come out to fix the problem, but the issue is you can’t flip a switch and get air traffic controllers here tomorrow.”