Washington DC - Construction work at the White House was a factor in the presidential helicopter passing too close to a passenger plane in the skies over Washington earlier this month, investigators said Thursday.

The close pass happened on August 4 after President Donald Trump took off on Marine One from a temporary site on the Ellipse, outside the grounds of the White House, during the construction of a permanent helipad inside the grounds.

The White House and the US Federal Aviation Administration (FAA) have both said that Trump was never in danger.

In their preliminary report, investigators at the US National Transportation Safety Board (NTSB) cited FAA estimates that the presidential helicopter and an Envoy Air passenger plane were separated by 0.82 miles horizontally and 700 feet vertically when the incident happened – less than the required minimum separation of 1.5 miles and 500 feet.

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The NTSB is working to "calculate and confirm" those distances and is continuing its investigation.

Investigators found that air traffic controllers at Ronald Reagan Washington National Airport did not receive the first radio transmission signaling the presidential helicopter's imminent departure and "the second attempt was broken and completely unreadable."

"There was not adequate line of-sight to ensure communications between the radio transmitter/receiver sight and The Ellipse, where (Marine One) was temporarily operating from while construction was ongoing at the White House," the report said.

In January, a collision between a passenger plane approaching the same airport and a US Army Black Hawk helicopter killed 67 people.