Investigators are focusing on whether two planes were visible to each other before colliding in Virginia in May.
Two small, private planes collided in mid-air on Memorial Day near Warrenton-Fauquier Airport in Fauquier County, killing two and injuring another.
The Transportation Safety Board of Canada is investigating the collision because the pilot of one of the planes was a National Transportation Safety Board employee and the other plane is owned by an employee of the Federal Aviation Administration.
The investigation is also trying to determine what role traffic control may have played, investigator Jon Lee said. Officials want to know what caused the crash to help prevent it from happening again.
A Beechcraft BE-35 plane and a Piper PA-28 crashed about a mile apart following the collision. The Beechcraft became engulfed in flames, and its passengers -- Paul Gardella, Jr., 57, of Burke, Va., and James M. Duncan, 60, of Bethesda, Md. -- were killed.
The pilot of the Piper -- 70-year-old Thomas R. Provin, of Broad Run, Va. -- was hospitalized with injuries. That plane was registered to an FAA employee.
Duncan, who owned the Beechcraft, was an NTSB doctor. Gardella was a flight instructor.