Building work at the White House contributed to radio problems that led to a flight being cleared for take-off as Donald Trump's helicopter was departing Washington, investigators have revealed.
The issue stopped air traffic controllers from hearing calls warning that the US president's Marine One was getting ready to take off, so they cleared a passenger aircraft to take off just as the helicopter took to the air, investigators said on Thursday.
And Marine One come within 0.8 lateral miles of an Envoy Air jet which was departing from Ronald Reagan Washington National Airport on 4 August.
Investigators were told that the crew of the civil airliner received a traffic warning from their collision avoidance system after they took off, though they never saw the helicopter.
Having both the jet and Marine One in the air at the same time also seemed to violate procedures put in place after last year's mid-air collision between a US Army Black Hawk helicopter and an arriving aircraft at the busy airport that killed 67 people.
After that incident, the Federal Aviation Administration (FAA) decided that all take-offs and landings at the airport would be halted when a helicopter passes by on a conflicting route.
The Pentagon also has a longstanding agreement with the FAA to provide controllers with a three-minute warning before the president's helicopter takes off that predates last year's crash.
Since mid-July, Mr Trump's helicopter has been lifting off from the Ellipse, a park south of the White House, because of construction work on a new helipad behind the White House. A new ballroom is also being built where the East Wing once stood.
Technicians determined after the incident that there was not an adequate line of sight between the radio receiver at the airport and the location where the helicopter takes off at the White House, the National Transportation Safety Board (NTSB) said.
To solve this, the relevant radios were moved from the neighbourhood where they had been located to the top of the tower at Ronald Reagan Airport.
Subsequent tests showed the radios working well after that change.
Officials also stressed that the president was never in danger.
The week before the president's helicopter got too close to the jet, air traffic controllers and Marine One pilots had met to discuss controllers not receiving the standard three-minute warning call, according to a preliminary report from the NTSB.
They made plans to relay radio calls through someone else if there were future problems, but that was also unsuccessful on the day of the incident.
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Communication issues came up again on Thursday, when air traffic controllers at the airport could not hear Marine One, and had to ask a support helicopter for help just before take-off.
"I can't hear Marine One," an air traffic controller says to the support helicopter. "If you hear Marine One, can you let me know?"
A little over a minute later, communication was functioning normally again for the afternoon flight to Joint Base Andrews outside Washington, according to recordings posted by ATC.com.
It was not immediately clear what caused the brief issue.
After last year's mid-air collision, it became clear that poor communication between air traffic controllers and the military had been a longstanding issue.
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