SALT LAKE CITY — A plane that slammed into an interstate highway in Utah started making strange sounds during takeoff and apparently struggled to stay at altitude before the crash that killed four people, according to a report from federal investigators.

The plane carrying four friends on vacation crashed four seconds after pilot Layne Clarke radioed to tell air traffic controllers he was going down, according to the preliminary report by the National Transportation Safety Board.

A final determination into what caused the crash could take more than a year, NTSB spokesman Eric Weiss said Thursday.

Flight mechanics at the airport in Ogden, Utah, told investigators that unusual sounds coming from the Beechcraft A36 Bonanza made them look up as the plane took off the afternoon of July 26, the report said. When the plane came into view, it was at least 400 feet (122 meters) lower than it should have been at that point, the witnesses said.

The engine sounded under-powered and the tail went up and down as if the pilot was struggling to keep the plane at altitude, the mechanics told federal investigators, according to the report released Monday.

Pilot Layne Clarke radioed air traffic control shortly after takeoff and said, “Hey, I’m going down,” the report states.

The controller cleared him for landing, but another pilot saw the small plane hit Interstate 15 four seconds later. It narrowly missed cars as it barreled across the lanes through a gap in traffic.

All four people on board were killed: Clarke, his wife Diana and two friends, Perry and Sarah Huffaker.

The group was traveling to Island Park, Idaho, for a vacation, friends have said. Clarke didn’t file a detailed flight plan, but was going to land at Yellowstone Airport in West Yellowstone, Montana, the report said.

A dash camera video from a car on a nearby street shows the plane starting with its wings level, then turning right and entering into a descending turn before it disappeared from the camera’s view.

The crash came 15 years after the pilot’s brother Corry Clarke died in a flight accident. He was a passenger learning to fly a gyrocopter, a hybrid of a helicopter and a plane, that went down while departing from the same municipal airport in Ogden, about 40 miles (64 kilometers) north of Salt Lake City.

The 2002 crash happened right after takeoff for a flight to drop off candy at a church children’s party.