The pilot of the balloon involved in the fatal accident in Virginia this month apparently tried to climb just before striking power lines , which caused the craft to catch fire, according to preliminary findings of federal investigators.

As the balloon approached the landing site in a field north of Richmond, the pilot “engaged the burner,” a preliminary National Transportation Safety Board report said.

“However, the balloon struck powerlines, which resulted in a spark,’’ the report said. Three people died in the May 9 accident.

“My guess is that he saw [the power line] at the last . . . and was trying to avoid it,” said Troy Bradley, past president of the Balloon Federation of America.

In hot-air balloon operations, a propane burner in the basket heats the air in the craft’s envelope, reducing its density, creating buoyancy and permitting ascent.

“The more you open the valve, the bigger the flame to heat the air, the faster the balloon rises,” according to the Web site.

After the spark was generated, the NTSB report said, “the balloon basket and a section of the envelope caught fire.” It said the balloon, with a 78,133 cubic foot envelope, began an accelerated climb and drifted out of the sight of witnesses.

The two passengers killed have been identified as Ginny Doyle, 44, associate head women’s basketball coach at the University of Richmond, and Natalie Lewis, 24, the team’s director of basketball operations. The pilot has been identified as Daniel T. Kirk, 65.

Debris was strewn along a 1.75 mile path, the report said, including two steel propane tanks, a fire extinguisher, the balloon’s instrument panel and pieces of charred fabric from the lower part of the balloon envelope.

Also recovered, the NTSB said, were three cellphones and a handheld GPS device.

Still missing, according to the report, are the burner and two propane tanks, as well as the balloon’s crown, crown ring and deflation port.

The Rappahannock Electric Cooperative said in a statement that it owned and operated the power lines involved. It said it was cooperating with the investigation.