SHERIDAN (AP) - A memorial service was held Friday for one of three people killed when a Wyoming Civil Air Patrol plane crashed during a search mission earlier this week, as investigators continue to look for a cause.

The memorial service for co-pilot James R. Meyer, 53, featured a helicopter and a fixed-wing aircraft flying over the service in tribute. A funeral was planned for Sept. 1 in Princess Anne, Md.

Meyer, a retired Army helicopter pilot from Sheridan, was among three members of the Civil Air Patrol's Wyoming Wing killed when their plane crashed in the Big Horn Mountains as they searched for a missing teen who was later found unharmed.

Also killed were pilot Lt. Col. James Henderson, 59, of Cowley and Capt. Patricia Larson, 52, of Sheridan.

Memorial services for Henderson and Larson were scheduled for Monday in Powell and Sheridan, respectively.

The investigation into what caused the crash posed a challenge since the Cessna 182R was destroyed by fire and had no voice and data recorders. There was also no communication from the plane indicating any distress.

"Right now, everything is fair game," said David Bowling, regional director of the National Transportation Safety Board.

Bowling said his investigator described the area where the plane went down as rugged, mountainous and "on the edge of a boxed canyon" with lots of trees in the area. He added people who saw the plane before the crash could provide important information.

"I was told there were a couple of firefighters that saw the airplane maneuvering prior to the accident occurring," Bowling said.

Investigators will observe the wreckage, which can help determine whether the plane was intact when it went down or broke up before crashing.

Even though the plane burned, investigators will examine parts for mechanical failure, Bowling said.

Weather and the pilot's actions will also be considered, Bowling said. An autopsy and toxicology report will likely be performed on the pilot, and investigators will examine maintenance and fueling records, he said.

The investigation could take a few months to a year, he said.