Four adults died Friday morning after two single-engine planes collided above a remote desert area in north Phoenix, authorities said.

The identities were not released pending notification of family members.

"This is a tragic event," Phoenix police spokesman Steve Martos said at the scene. "It could have been much worse and be in a congested area where people reside."

Martos said planes frequently fly in the area because of the open airspace.

A pilot reported seeing the two small aircraft collide in midair about 15 miles northwest of Deer Valley Airport, spokesman Ian Gregor of the Federal Aviation Administration said.

Numerous 911 calls about the midair crash came in around 10 a.m., authorities said.

Fire crews went to the area and found the two planes. One plane, believed to be a Cessna, caught fire upon impact and was "unrecognizable," according to Capt. Dave Wilson of the Daisy Mountain Fire Department.

He said the plane contained two people, whose gender could not be determined at the scene, because the bodies were burnt.

Identification of these two victims will be made by the Medical Examiners Office, Martos said.

Martos said the identities of the two men in the other aircraft are known but the family has yet to be notified.

Wilson said the other aircraft, a Piper Archer III, appeared to have made a rough landing and was mostly intact. It was about 100 yards from the other plane with the two victims inside, he said.

"I thought possibly we might have survivors," said battalion Chief Gary Bernard of the Peoria Fire Department.

The Phoenix Police Department is investigating the cause of the crash, along with investigators from the Federal Aviation Administration and the National Transportation Safety Board.

An NTSB preliminary report is expected to be posted on the agency's website, www.ntsb.gov within a week or two, Gregor said. He said it typically takes NTSB months to come up with a probable cause for the collision.

The Cessna is part of Westwind School of Aeronautics at Deer Valley Airport, Martos said.

The Piper is owned by Bird AcquisitionLLC., which operates TransPac Aviation Academy, a local flight school that trains U.S. and international pilots. Bird Acquisition is a Massachusetts company with an office location at the Deer Valley Airport.

TransPac says on its web site that it has a fleet of 60 Piper planes, which are maintained by FAA-certified pilots. At least two additional TransPac planes have been involved in fatal crashes in recent years.

Stephen Goddard, the company's CEO, did not return calls seeking comment. The company also declined comment when a reporter visited its Deer Valley campus.

Deer Valley Airport is one of the busiest general aviation airports in the country and has more than 1,200 planes based there.

Typically, Arizona's fatal crash rate, measured by deadly crashes per 100,000 hours of flight, is well above the U.S. average.

Arizona was double the national rate in 2008, at 2.76 per 100,000 flight hours.

Causes are myriad, and safety experts have no clear explanations why Arizona consistently records a higher fatal-crash rate for light aircraft than the U.S. average.

Source: The Arizona Republic