(CNN) -- The pilot of a plane that crashed last month in Montana twice requested a change of destination in radio comments to air traffic controllers but "gave no reason for the diversion," according to a preliminary report from the National Transportation Safety Board issued Thursday.
A witness in Butte, Montana, took this photo shortly after the plane crash on March 22.
Fourteen people were killed when the plane slammed into a cemetery March 22 in Butte, and investigators immediately began focusing on why the pilot asked to divert the flight from Bozeman, Montana, to Butte, 80 miles to the west.
The report said the single engine-Pilatus PC-12/45 was carrying people to a ski vacation.
The plane originally departed Redlands, California, for Nut Tree Airport in Vacaville, California, where passengers were picked up. It then went to Oroville, California, to pick up more passengers.
The pilot left Oroville at 12:10 p.m. local time, with Bozeman as the destination and Butte as the alternate, according to an instrument flight rules flight plan the pilot filed.
The pilot requested to divert to Butte at 2:03 p.m. and "was cleared at pilot's discretion to descend to 14,000 feet." The pilot two minutes later again requested to divert to Butte.
The report said that at 2:27 p.m., controllers asked the pilot whether he had the airport in sight and "the pilot indicated he had one more cloud to maneuver around." A minute later, the pilot reported the airport in sight and controllers "terminated radar service."
At 2:29 p.m., air traffic control "called the aircraft in the blind with no response," and the accident was reported four minutes later.
The aircraft slammed into a cemetery about 500 feet short of the Butte airport.
"Initial reports from ground witnesses indicate that the airplane was flying approximately 300 feet above ground level in a north-northwesterly direction. Shortly thereafter, the airplane's nose pitched to a nose-low attitude and it impacted the ground. One witness with aviation experience reported that the airplane was west of the runway centerline and appeared too high to land on the runway," the report said.
"The witness then saw the airplane bank to the left and fly farther west when it rolled, pitched down and descended out of his view. Although there is no air traffic control tower at Butte, the local fixed base operator lineman was monitoring the radio as the airplane approached the airport. He heard the pilot transmit that he would be landing on runway 33."
Investigators also were looking into whether the single-engine plane was overloaded and what role weather may have played in the accident.
The preliminary NTSB report said "visual meteorological conditions prevailed" at both Bozeman and Butte.