An airplane crash in Chester County that ended in two deaths Sunday came during a routine flight review conducted every two years to test pilots' skills, officials said Monday.

The two victims aboard the flight were both certified pilots, officials said. The pilot flying the fixed-wing, single-engine airplane was accompanied by a certified flight instructor with Federal Aviation Administration designations.

On Monday, 24 hours after the crash, many questions remained - mainly, what caused a small plane to fall abruptly from the sky Sunday after traveling just two miles from the Brandywine Airport in West Goshen Township.

Federal officials who were at the scene, including representatives from the FAA and the National Transportation Safety Board (NTSB), along with the West Goshen Police Department, said an investigation is ongoing.

Officials are meeting with representatives of the engine and airplane manufacturer, investigating who maintained the plane last, and inspecting the fuel supply and engine, said Tim Monville, a senior air safety investigator with the NTSB.

"An engine malfunction - whether it be catastrophic or a minor power loss - does not equate to an aerodynamic loss of control," Monville said at a news conference. As long as certain airspeed is maintained, among other factors, "the airplane still should be capable of flying."

"Based on the account of a sputtering engine, we will check the fuel-supply system," Monville said. He could not confirm whether the plane stalled in the air.

Much of the airplane was destroyed in the crash, which may make some inspections more difficult, Monville said.

According to eyewitnesses at the airport, Monville said, a routine engine check was performed before the Sunday flight. After being cleared, the airplane proceeded to take off, and witnesses heard the engine begin to "sputter," and then subsequently heard the engine's power restored.

Seconds later, the engine sputtered again before witnesses lost sight of the plane, Monville said.

Police responded to the crash shortly after 1:30 p.m. at the backyard of a residence along the 1000 block of Saunders Lane, in a wooded open space along the block, which is about two miles from the airport.

When police arrived, the cockpit, cabin and wings were "consumed" by a postcrash fire, officials said. Officials could not say who owned the plane. Brandywine Airport officials and neighbors declined to comment.

Monville said officials are gathering security video from a nearby facility that captured the final portion of the flight. No information has been identified from the airplane's radio.

Family members of the flight instructor gathered at the scene Monday morning. The names of the victims will not be released until later this week after autopsies are completed, which were scheduled for Monday afternoon. A preliminary report on the crash will be published next week, Monville said.