NEW YORK (CNN) -- Investigators resumed pulling debris from the Hudson River Sunday as they searched for clues to the cause of Saturday's fatal collision between a small plane and a helicopter over the river.
"We're very interested in any witnesses that might have any video, surveillance video or other footage to come forward," National Transportation Safety Board Chairwoman Deborah Hersman told CNN.
Divers working in murky water up to 50 feet deep gingerly pulled objects from beneath three orange buoys that marked where the helicopter carrying a pilot and five Italian tourists plummeted into the water after colliding with a plane carrying three people.
Autopsies on the three bodies recovered Saturday were to be carried out Sunday; none of the nine people aboard the two aircraft are believed to have survived.
At about 8 a.m., divers brought up a fourth victim, then placed the body into a bag and lifted the bag into a police boat. New Jersey State Police Sgt. Stephen Jones confirmed the find and said a large piece of the helicopter's fuselage had been recovered as well.
Hersman said investigators will focus on radio communications along the congested air corridor at the time of the crash.
"We're looking very closely at any air-traffic control communications that might have occurred between the tower and the fixed-wing aircraft that had just taken off from Teterboro," she said, referring to the nearby New Jersey airport.
Witness accounts and still photographs had already provided "good information" to help investigators put together the sequence of events, she said. Still, she added, "We've got a lot of work ahead of us."
Of the three bodies found Saturday, one was recovered from the plane -- a Piper Saratoga PA-32.
The victims in the plane have been identified as: Steven Altman, the pilot and owner of the plane, his brother, Daniel and nephew Douglas, a source involved in the investigation said. The body recovered is believed to be that of the nephew.
The other two bodies pulled out are believed to be two tourists aboard the helicopter, a law enforcement source said.
The helicopter was carrying a pilot and five people visiting New York from Bologna, Italy.
The tourists were part of a group of 12 Italians visiting the United States. The helicopter was operated by Liberty Helicopter Sightseeing Tours.
The helicopter wreckage was found in about 30 feet of water, while the plane is believed to be near the midchannel point of the Hudson in deeper water, the source said. A side-scanning sonar is being used to pinpoint the plane and has identified possibly a third debris field, the source said.
A witness told investigators he saw the airplane approach the helicopter from behind, and the plane's right wing made "contact with the helicopter," Hersman said.
The witness, another Liberty pilot who was refueling at a nearby heliport, said he tried to warn the helicopter pilot, but got no response.
"This is a VFR corridor -- that means Visual Flight Rules prevail," Hersman told reporters late Saturday. "You are supposed to be alert, and see and avoid other aircraft in the vicinity."
Witnesses reported seeing debris flying from the helicopter as it slammed into the water.
Arnold Stevens, who saw the collision from the W Hotel in Hoboken, New Jersey, said the helicopter "dropped like a rock." After one of the plane's wings was sheared off, it began "corkscrewing" into the water, he said.
The busy airspace surrounding Manhattan has been the site of several recent aeronautical mishaps.
Earlier this year, a US Airways plane with 155 people on board ditched into the Hudson after apparently striking birds upon takeoff from New York's LaGuardia Airport, officials said.
Capt. Chesley B. "Sully" Sullenberger's landing, which resulted in no deaths or serious injuries, was captured on closed circuit television.
In 2006, Yankees pitcher Cory Lidle, 34, and his flight instructor were killed when the ballplayer's plane crashed into a high-rise apartment building near the East River, city officials said.