Federal investigators probing a midair collision between two medical helicopters near Flagstaff Medical Center were at the scene Tuesday assessing what pieces of wreckage they should collect for further analysis in Washington.

The effort comes two days after the choppers collided as they approached the hospital, killing three people on each and leaving a flight nurse in critical condition.

A spokesman with the National Transportation Safety Board said investigators will carefully document the wreckage of the two Bell 407 helicopters and photograph the scene. They continue to conduct interviews with witnesses.

The sole survivor of the crash is James Taylor, 36, of Salt Lake City. He remained in critical condition at the Flagstaff hospital Tuesday morning.

NTSB investigators arrived in Flagstaff on Monday to begin the monthslong process of looking for a cause. They are examining the wreckage and they will also look at the video from a surveillance camera from a hospital parking lot that captured the crash, but it will require technical work to remove the time stamp that blocks footage of the collision.

The collision was only the second midair crash involving a medical helicopter in the past 25 years, and the first that involved two medical aircraft, said Ian Gregor, a spokesman for the Federal Aviation Administration. It was the ninth accident this year involving emergency medical aircraft, bringing the number of deaths to 16, NTSB officials said.

One of the helicopters in the Flagstaff collision was coming from the South Rim of the Grand Canyon, about 70 air miles from Flagstaff, farther by ground. The other chopper was coming from Winslow, about 50 miles away on Interstate 40.

The patient on the helicopter from the Grand Canyon was a firefighter who had suffered anaphylactic shock — a life-threatening allergic reaction — from a treatment he received for a bug bite, authorities said. No information was released on the other patient's injuries.

Three people died on each of the two helicopters: the pilot, the patient and a flight nurse on one chopper, the pilot, the patient and a paramedic on the other.