The U.S. National Transportation Safety Board on Thursday released its preliminary report on the Aug. 4 loss of separation event involving a Marine One presidential helicopter and a commercial airliner departing Ronald Reagan Washington National Airport (DCA), suggesting that blocked radio transmissions associated with the helicopter’s temporary operating location played a role in the incident.

The Sikorsky VH-3D helicopter and an Embraer E170 operated by Envoy Air came within 0.82 miles of each other laterally and about 700 feet vertically, according to a preliminary estimate. The Envoy Air pilots told investigators they received an associated traffic advisory from their traffic collision avoidance system (TCAS), but not a resolution advisory that would have required maneuvering to avoid the helicopter.

Marine One pilots are required under a letter of agreement to provide the DCA air traffic control tower with a radio call three minutes prior to departure, at which point controllers should halt all departures from the airport. The NTSB report indicates that the helicopter pilots — who were temporarily operating from The Ellipse rather than the White House south lawn due to construction — attempted several times to make the call, but it did not go through.

A subsequent coverage analysis by FAA technicians from Spectrum Engineering determined there was not adequate line-of-sight to ensure communications between the radio transmitter/receiver and The Ellipse. The radios for the helicopter control frequency have since been relocated from a nearby residential neighborhood to the top of the DCA air traffic control tower, the report states.