The Cessna 172 plane crash that killed three people, including two South King County swim coaches, smashed into trees on the western face of Mount Si in North Bend during a “personal sightseeing flight” about 20 minutes after takeoff from Renton Municipal Airport.

The victims of the Feb. 15 crash are Federal Way residents Robert Hill, 30, and Elizabeth Redling, 29, along with Seth Dawson, 31, a swim coach at Kentlake High School in Kent. Hill was a swim coach at Decatur High School and the men coached the Valley Aquatics Swim Team in Federal Way.

“The airplane fragmented upon impacting trees and up-sloping mountainous terrain, which resulted in substantial structural damage,” according to the preliminary report released Tuesday by the National Transportation Safety Board. The fuselage was found upside down.

The three victims were on a “personal sightseeing flight,” according to the report. The plane crashed at about 1:54 a.m. after leaving Renton at about 1:35 a.m.

The NTSB expects to release a probable cause report by the end of the year, said Wayne Pollack, NTSB crash investigator, during a Tuesday phone interview.

“The investigation is still continuing,” Pollack said. “It takes several months to do a final report when we will issue a statement of probable cause when the investigation is completed.”

Pollack said examination of the crash site and aircraft are complete. The ongoing investigation will include toxicology reports, additional records on the pilot Hill and further interviews with witnesses.

Dawson was in his second season of coaching the Kentlake boys swim and dive team. He coached the Kentlake girls team in the fall. Hill had been the Decatur swim coach for five years.

According to the preliminary report, Christiansen Aviation Inc. of Wilmington, Del., the plane’s registered owner, leased the plane to AcuWings, a Renton flight school. Hill had a commercial pilot license and held a certified flight instructor certificate.

The flight originated from Renton Municipal Airport, but no flight plan was filed. Pollack said there is no law that requires a flight plan for a sightseeing personal flight.

A review of Federal Aviation Administration (FAA) recorded radar showed the aircraft had an initial climb out of Renton to 2,400 feet mean sea level as indicated by its altitude encoding transponder.

“Initially, the aircraft proceeded in a northeasterly direction,” according to the preliminary report. “However, as the aircraft approached Snoqualmie Falls, it descended to 1,500 feet and proceeded on an east-southeasterly course. The last radar hit occurred at 1:46 a.m., at which time the aircraft was about 1 mile southwest of the falls, and about 1 mile north of Interstate 90. During the last minute of recorded flight, the aircraft’s ground speed decreased from about 112 to 106 knots.”

A witness, who holds a private pilot certificate, told investigators that he was driving along I-90 at about 1:50 a.m. when he saw the plane.

“He reported having observed the anti-collision and navigation lights from a low flying airplane that was cruising in a southeasterly direction an estimated 1 mile north of I-90,” according to the report. “The witness stated that when I-90 turned southeasterly, he lost sight of the airplane for a couple of minutes. However, he regained visual contact with the airplane as he and the airplane approached North Bend. At that time, the airplane had altered its course and was heading in a northeasterly direction.

“The witness estimated that the airplane’s altitude was about 1,000 feet above ground level. (North Bend’s elevation is 400 to 500 feet mean sea level). The witness stated that the visibility was at least 3 miles. There was an overcast ceiling several thousand feet above the ground, with a few lower elevation clouds. Based upon the flight path drawing provided by the witness, the Safety Board investigator notes that when the witness lost visual contact with the airplane, it was flying toward the Mount Si area and was within 3 miles of the crash site.”

At about 1:54 a.m., several people in North Bend called 911 to report having heard an impact sound.

“At least one witness reported having observed the lights of a low flying airplane and the sound of its engine suddenly stop following its 1:53 a.m. low altitude easterly direction flight over the city toward Mount Si,” according to the report.

Pollack’s investigation of the accident site and plane wreckage “revealed evidence of multiple broken tree trunks and felled branches on the mountainside in Mount Si’s Natural Resource Conservation Area.”

Pollack also wrote in the report about examination of the plane.

“Fragmented airframe components, including both crushed wings, were noted below dozens of felled branches on an approximate 120-degree magnetic track leading to the fuselage, which was upside down,” according to the report. “No evidence of pre-impact oil leaks, fuel filter blockage, flight control anomalies, or fire was noted at the estimated 1,950-foot mean sea level crash site.”