Washington (CNN) -- The pilot of a small plane that collided with a sightseeing helicopter over New York's Hudson River last summer had scant seconds to see the helicopter as it rose to his altitude, but the plane pilot apparently attempted evasive action immediately before the collision, NTSB documents released Wednesday show.
Computer-generated images show the helicopter appeared as a tiny spot, perhaps lost among the backdrop of Manhattan buildings, in the seconds before the crash, which killed nine people. It would have been difficult for the plane's pilot to see the helicopter "until the last instant," one National Transportation Safety Board report says.
And since the Piper PA-32 was above and behind the slower moving helicopter, "it is not likely that the airplane would have been visible to the pilot of the helicopter," the NTSB said.
All three people on the private plane and all six aboard the sightseeing helicopter died in the August 8, 2009, crash.
The two aircraft collided over the Hudson River, a heavily trafficked corridor separating Manhattan and Hoboken, New Jersey.
Pilots of both aircraft were operating under visual flight rules, which requires aircraft pilots to "see and be seen." But the pilot of the plane also was receiving "flight following" radar service from Air Traffic Control, intended to help keep aircraft separate.
The NTSB is reviewing, among other things, the actions of two air traffic controllers, one of whom was involved in a personal phone call at the time of the mishap, and another who was running a personal errand. Both controllers were placed on administrative leave after the collision.
Three months after the crash, the Federal Aviation Administration also tightened control of the airspace, separating low-altitude local aircraft flights over the Hudson River, such as a sightseeing helicopter, from flights transiting through the river airspace, such as the small aircraft.
The new regulations create an exclusion zone for pilots who are over the Hudson River below 1,300 feet, in which they "must announce their aircraft type, position, direction and altitude at charted mandatory reporting points, and must stay along the New Jersey shoreline when southbound and along the Manhattan shoreline when northbound," the FAA said.
The incident occurred shortly before noon on a clear day. The airplane, with a pilot and two passengers, departed Teterboro Airport in New Jersey at 11:49 a.m. headed for Ocean City, New Jersey. Three minutes later, the helicopter, operated by Liberty Helicopters, departed West 30th Street Heliport.
The plane was eight miles from the airport and had leveled off at about 1,100 feet, flying at 145 and 155 knots (about 167 to 178 mph), at the time of the collision. The helicopter was climbing and was probably at a ground speed between 90 and 95 knots (about 104 to 109 mph). They had a closing speed of 70 knots (about 81 mph).
An analysis of video taken by a passenger on a ferry boat indicates that the plane "suddenly steepened its roll to the right, suggesting the beginning of an evasive maneuver," the NTSB said.
The NTSB said its computer depiction of the collision shows the helicopter would have remained a relatively small object in the plane's windshield until about five seconds before the collision. Until the last instant, the chopper would have appeared below the horizon and against a background of buildings.
It is not known whether either aircraft received audible alerts about the proximity of the other aircraft, though it is likely the helicopter received an alert.
The NTSB on Wednesday released information about the crash, but it will not determine a probable cause for the accident until later this year.