A single-engine, Beech 19A plane that fatally crashed in the Waianae Mountains above Kunia was traveling at an altitude of 2,000 feet before it disappeared from radar, according to a preliminary report by the National Transportation Safety Board.

The federal agency released the report Tuesday as they continue their investigation into the cause of the July 28 crash that killed passengers Gerrit Evensen, 28, his girlfriend Heather Riley, 27, Alexis Aaron, 32 and pilot Dean Hutton, 29. Their bodies were recovered from the wreckage near Palikea and Pohakea Pass trails the following day.

A final report on the crash is expected to be released in 12 to 18 months.

At about 6:37 p.m. on July 28, the four friends departed Daniel K. Inouye International Airport on the four-seat, fixed-wing plane for a sunset flight. According to the preliminary report, the plane was traveling at an altitude of 1,800 feet before it made “descending and climbing turns during the next 6 minutes.”

During the last minute, the plane climbed and was at 2,000 feet when the plane disappeared from radar. The report said the plane crashed into terrain at about 6:52 p.m. Friday.

The Federal Aviation Administration issued an alert notice shortly after 1:20 p.m. Saturday when family members reported the flight was overdue.

A Coast Guard MH-65 Dolphin helicopter spotted the wreckage on steep terrain at about 3 p.m. in the area where the plane was last detected.

NTSB Preliminary Plane Crash Report, July 2017 by Honolulu Star-Advertiser on Scribd