An air traffic controller making a personal phone call initially failed to warn a small plane of a helicopter in its path and then tried unsuccessfully to contact the pilot moments before the two collided in mid-air over New York's Hudson River, U.S. safety officials said Friday.
The National Transportation Safety Board said a controller at a Teterboro airport in New Jersey cleared the plane for takeoff last Saturday and then made the personal call.
The board said radar data show that there were several blips in the plane's path, including the helicopter.
The board said it was not until controllers at nearby Newark airport alerted the Teterboro controller to the potential collision that he tried unsuccessfully to contact the pilot.
Five Italian tourists and four other people died in the crash over the busy waterway between Manhattan and New Jersey.
A witness's video recording shows the small plane, which was flying at about 330 metres, approached the Liberty Helicopter Tours aircraft, which had just taken off for a 12-minute tour over New York City, and appeared to clip it with a wing.
Both aircraft split apart and fell into the river, scattering debris.
The FAA said Thursday night it has begun disciplinary proceedings against the air-traffic controller and a supervisor, who was on duty at the time but not in the building as required. Both have been placed on administrative leave.