"We're very concerned about that," National Transportation Safety Board chairman Mark Rosenker told reporters. "We're going to work very, very hard to make sure we understand exactly what happened here ... and make recommendations to prevent it from happening again."
The collision Sunday near Flagstaff Medical Center killed both patients, critically injured a nurse, and rained debris near a residential area. Two emergency workers suffered minor burns in an explosion in the wreckage of one of the aircraft.
Lawrence Garduno, who lives about a half mile from the crash, said he heard a loud boom that rattled the windows. He drove toward the hospital and stopped to see the burning wreckage. "It kind of scares me," Garduno said. "If this had happened a half mile closer, it could have fallen on our house."
Witnesses said the weather was clear, and neither helicopter seemed to have mechanical problems, CBS News reports.
There have been nine serious accidents with emergency medical aircraft this year, six of them involving helicopters, NTSB spokesman Ted Lopatkiewicz said. Sixteen people died in the helicopter accidents, including the six in Sunday's crash, which had the largest death toll. Rosenker called the numbers a "disturbing trend."
One of the helicopters that crashed Sunday was carrying a patient with a medical emergency from the Grand Canyon, officials said. It was operated by Classic Helicopters of Woods Cross, Utah.
The other helicopter, operated by Air Methods from Englewood, Colo., was coming from the nearby community of Winslow, said Capt. Mark Johnson, a spokesman for the Flagstaff Fire Department.
The victims from the Classic helicopter were identified as pilot Tom Caldwell, 54, paramedic Tom Clausing, 36, and the Grand Canyon patient, Michael McDonald, 26. Flight nurse James Taylor, 36, was in critical condition Monday at Flagstaff Medical Center.
On the other helicopter, the victims were identified Monday as pilot Pat Graham, 50, flight nurse Shawn Shreeve, 36, and patient Raymond Zest, 54.
"We've been in business 20 years, and these are the first fatalities we've experienced," said Matt Stein, a pilot with Classic Helicopters subsidiary Classic Lifeguard Aeromedical Services in Page, Ariz. "They were all heroes. They were out doing a great service for their communities."
Aaron Todd, chief executive for Air Methods Corp., said Monday that his company's helicopter was being flown by a veteran pilot. Citing the ongoing investigation, he declined to discuss details.
Flagstaff Medical Center President Bill Bradel said Monday the hospital was not releasing any details on the crash, instead focusing on family members of those who died.
"I watched our trauma team work diligently trying to save the lives of the victims of this terrible crash," he said.
Both aircraft were Bell 407 models, said Ian Gregor, a spokesman with the Federal Aviation Administration.
Stein said it was rare for two medical helicopters to attempt to land at a hospital at the same time. Flagstaff Medical Center doesn't have flight controllers, he said, and it's up to the pilots to watch each other as they approach.
The falling debris set off a brush fire, which spread to 10 acres before it was contained. The aircraft were so mangled, Johnson said, that "they're not recognizable as helicopters."
It was the second major helicopter collision in Arizona in less than a year. Last July 27, two news helicopters collided while covering a car chase near Phoenix, killing all four people on board. Rosenker said NTSB investigators will look at similarities between the two accidents to identify any potential safety gaps.
Flagstaff is about 130 miles north of Phoenix.