WATCH: Communication problem behind mid-air plane collision in St Bruno?

Was a communication problem the reason for the plane collision in the skies above south shore St Bruno?

Aircraft safety regulators are trying to piece together the circumstances of Friday's mid-air crash involving two small planes.

In newly released audio, an air traffic controller at Saint-Hubert airport can be heard instructing one pilot to stay at a certain altitude, while also noting another plane is ahead and below.

The tower attempted to contact the plane four times without getting a response.

One pilot, 21 years old, was killed and the other, 23, suffered non-life threatening injuries after the two planes hit and came crashing down.

One plane fell onto the roof of the Promenades St Bruno shopping center, the other onto the parking lot. 

No one on the ground was hurt, but two witnsses were treated for shock.

One witness recalled a scene of shredded metal and noted that some people started crying when they realized what had happened.

Both planes were Cessna 152's operated by Cargair, a pilot-training academy based in nearby Longueuil.

The school said it was co-operatng with authorities.