In audio released by flight tracking company FlightAware, the pilot of a private plane communicates with air traffic control on Thursday before colliding with a helicopter conducting a training exercise in midair near Frederick Municipal Airport. (FlightAware)

Federal investigators were trying to determine Friday what caused a collision involving a helicopter and a plane that killed three people Thursday afternoon in Frederick County.

National Transportation Safety Board officials spent the day sifting through the wreckage at the crash site near Frederick Municipal Airport and interviewing survivors and air traffic controllers.

The crash involved a helicopter owned by Advanced Helicopter Concepts, a Frederick company that routinely trains pilots at the airport. The helicopter was on a training exercise when it and a private plane collided in midair about 3:40 p.m. Thursday.

NTSB investigator Brian Rayner said Friday that he did not know of any mechanical problem affecting either aircraft at the time of the collision.

The three people who died were identified as William Jenkins, 47, of Morrison, Colo.; Breandan J. MacFawn, 35, of Cumberland, Md.; and Christopher D. Parsons, 29, of Westminster, Md. All three were on board the helicopter, according to investigators and an Advanced employee.

Families of the victims declined to discuss the collision or could not be reached for comment Friday.

Parsons was a flight instructor employed by Advanced, according to Chris Hollingshead, a pilot for the company. Hollingshead said Parsons was taking Jenkins on a test flight when the crash occurred. Jenkins had rented other aircraft from Advanced, he said, but had never piloted one of Advanced’s Robinson R44 helicopters.

The company requires pilots to take about a half-hour test flight with an instructor on a prescribed course around the airport before renting a new type of aircraft, Hollingshead said. Jenkins probably would have been at the controls, under Parsons’s supervision, at the time of the collision, he said.

Hollingshead said MacFawn was a passenger and was not employed by Advanced.

Two Maryland men suffered minor injuries in the crash and were treated at Meritus Medical Center in Hagerstown and released Thursday evening, Maryland State Police said in a statement. Both men — Gilbert Porter, 75, of Sandy Spring and Scott V. Graeves, 55, of Brookeville — had been in the plane, said police spokesman Greg Shipley.

Federal Aviation Administration spokeswoman Laura Brown said the plane was a 2006 Cirrus SR22, a four-seater equipped with a parachute that can be deployed for landing in an emergency.

Records indicate that the plane, which is registered in Olney, Md., was flying to the Frederick airport from Cleveland, Tenn. Three helicopters were in the air as the plane approached.

Porter said late Thursday that he and Graeves had clearance to land and were in their pattern to descend when they suddenly felt the collision. In a matter of seconds, both Porter and the pilot knew they were going to crash.

“We were heading toward the ground at 60 miles per hour. I mean, it was scary as hell,” Porter said in a telephone interview.

After the aircraft collided, Graeves deployed the parachute, which slowed the plane until it careened into a small tree, breaking the plane’s momentum.

“I’m very lucky to be alive,” Porter said.

Hollingshead, serving as a spokesman for Advanced, said the company has voluntarily suspended all operations during the NTSB investigation. He said that Advanced’s 10 aircraft normally make 30 to 40 flights per day and that having two or three helicopters in the air while planes are also using the airport is standard.

Hollingshead said the helicopter was at altitude, neither taking off nor landing. Rayner said that the positions and altitudes of the two aircraft remained unclear as investigators gathered data Friday.

A Robinson R44, also owned and operated by Advanced, was involved in a crash in Western Maryland in 2009 that killed four people.

Dana Hedgpeth, Ashley Halsey III, Clarence Williams and John Woodrow Cox contributed to this report.