CHEYENNE, Wyo. (AP) — The pilot of a single-engine airplane was struggling to get over Wyoming’s highest mountains in bad weather before it crashed and killed the pilot and three of his young sons, a voice recording shows.

Luke Bucklin of Minneapolis said in a radio call to ground controllers just before the crash that his aircraft might not reach 16,000 feet, the altitude he was assigned by an air traffic controller.

“Descending rapidly,” the pilot says on the recording.

“Reporting severe mountain waves,” he said about a minute later, referring to powerful up-and-down wind currents over the peaks. “Probably going to (garble).”

It was the last audible transmission from Bucklin on the recording obtained from the Federal Aviation Administration by The Associated Press through a Freedom of Information Act request.

Preliminary investigative reports indicate the last transmission came around the time the plane crashed Oct. 25 in the Wind River Range of northwest Wyoming.

Bucklin, 40, was killed, along with his 14-year-old twins, Nate and Nick, and 12-year-old son, Noah. They were on their way home from a wedding.

A weeklong search located their bodies and the wreckage on a mountainside at about 11,100 feet. They were six miles southeast of Gannett Peak, Wyoming’s highest point at just over 13,800 feet.

National Transportation Safety Board investigators haven’t disclosed the cause of the crash.

It was snowing heavily when the Mooney M20J took off from Jackson, Wyo., airport manager Ray Bishop said. It crashed less than an hour later.

A preliminary NTSB report said Bucklin radioed about 41 minutes after takeoff that the plane had a trace of ice and had encountered light turbulence.