The pilots of two Phoenix news helicopters had so many distractions they lost track of each other, resulting in a midair crash that killed four, a federal safety panel said Wednesday.

National Transportation Safety Board said the probable cause of the July 2007 crash was that the two helicopters drifted too close together because the pilots were focused on watching a police car chase and reporting the event for their television stations.

The five-member board voted unanimously to recommend that TV stations no longer have helicopter pilots also report news while they fly unless the stations can demonstrate pilots can do both safely.

Investigators told board members that in addition to flying and reporting, Phoenix pilots were also listening to police radios, coordinating coverage with news producers, talking with photographers on board, talking to other helicopter pilots by radio and trying to visually keep track of other helicopters nearby. The two pilots and two news photographers on board were killed when the helicopters collided and then plunged into a city park.

There were four news helicopters and a police helicopter following the chase at the time of the crash, and a fifth news helicopter was en route, NTSB investigators said.

It was the only midair crash of two news helicopters in the United States, NTSB staff said. But board member Kitty Higgins said at least 18 other incidents in which two news helicopters nearly collided in midair have been reported to a voluntary aviation safety database managed by NASA.

"We've had a lot of near misses, if that's a fair way to characterize it," Higgins said. She noted those reports were filed by pilots and others who were concerned enough to file a report.

"We don't really know how many other near misses may be out there," she said.

Board members said they didn't want to impede news coverage, but they are concerned about the possibility of future accidents, particularly involving coverage of breaking news stories like car chases. They said news organizations often share a helicopter for coverage of planned events like a professional football game and the risk is much lower.

One of the Phoenix stations involved in the crash now has a second pilot on board to do the reporting, while the other station no longer has pilots report as they fly, investigators said. But other Phoenix stations haven't made similar changes, they said.

That concerned board member Debbie Hersman, who said that if some of the stations most familiar with the Phoenix accident weren't changing their practice of reporting by pilots then it's unlikely stations elsewhere will make changes without prodding by the NTSB or the Federal Aviation Administration.

"I think we need to put the obligation on them (the television news industry) to show how combining these responsibilities is safe and, if necessary, to assign the reporting duties to someone else. But if you can demonstrate it can be done safely, that's fine," Hersman said.

"This accident is an early warning sign for us — it's the canary in the coal mine," Hersman said. "It showed very clearly these pilots got distracted, and if they got distracted other pilots could get distracted. ... I don't think those four lives lost in Arizona were worth that cops-and-robbers drama."

The board also recommended news helicopters be equipped with a new type of brighter lights and be painted in a manner that makes them more visible to other helicopters. Other recommendations included development of horizontal and lateral separation requirements and audio, visual and data recorders for helicopters.