Four people have died after two helicopters, owned by rival US television channels, collided in mid-air while covering a car chase.Both helicopters went down in Steele Indian School Park in Phoenix, Arizona, and caught fire as they were covering the police pursuit of a pick-up truck.It is thought all those on board were killed. No-one on the ground was hurt.Within moments of the collision, other TV news helicopters were broadcasting pictures of the burning wreckage.Viewers did not see the accident as both cameras were pointed towards the ground.
