FLAGSTAFF -- Federal investigators are trying to determine what caused two medical helicopters to crash in mid-air Sunday afternoon, killing six people and critically injuring a seventh Sunday afternoon.

Footage taken moments after crash

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All three people on one of the helicopters were killed, including a patient and the pilot, said Ian Gregor, spokesman for the Federal Aviation Administration.

Three others aboard the second helicopter were killed and one critically wounded, Gregor said. The injured flight nurse remains in critical condition.

The helicopters were about 1/2 mile from Flagstaff Medical Center when they collided. Authorities said both helicopters were headed into the medical center when they collided.

According to the FAA, both helicopters were Bell 407 models: one of them operated by Air Methods out of Englewood, Colo., and the other operated by Classic Helicopters of Woods Cross, Utah. Neither company returned calls from The Associated Press Sunday.

One of the choppers went down in an open field. The other crashed in a wooded area.

The remains of the helicopters will be brought to Phoenix where investigators will try to piece them together as part of their investigation.

The chairman of the National Transportation Safety Board said they found that the tail of one helicopter showed signs that it had been hit by a rotor blade.

The three people on the Air Methods aircraft were identified as the pilot, Pat Graham, 50; the flight nurse, Shawn Shreeve, 36; and patient Raymond Zest, 54, who was being flown in from Winslow.

The three onboard the Classic Lifeguard helicopter were identified as the pilot, Tom Caldwell, 54; paramedic Tom Clausing, 36; and patient Michael MacDonald, 26. MacDonald was a firefighter who was hospitalized last week with an insect bite while fighting the Walla Valley fire.

An injured flight nurse on the Classic helicopter, James Taylor, 36, remains in critical condition.

Nobody on the ground was hurt when the choppers went down, although three first responders were burned when a fuel tank exploded on the ground. Those people are expected to recover.

The fire from the crash and explosion touched off a 10- to 15-acre fire. Fire crews were able to get that under control relatively quickly.

FAA officials and the NTSB are still on site to try and determine what happened.

Anyone who witnessed the crash is asked to call 928-774-1414.

NTSB chairman Mark Rosenker said the fatal collision was the 10th crash involving emergency medical aircraft this year. Three people were thrown from an Air Evac helicopter when it crashed outside Ash Fork Friday morning. ( Read the story.)

Rosenker also said that officials will look at similarities between the Flagstaff collision and last year's crash involving the news helicopters of Channel 3 and Channel 15 to identify any potential safety gaps. Four people died in that crash.

The Associated Press contributed to this report.