A wing from a Cessna 208B broke away before the plane crashed Friday into a field northeast of Seattle, killing the four people on board, the National Transportation Safety Board said on Monday.
The NTSB said the wreckage has been recovered and taken to a facility for reconstruction, The Seattle Times reported.
A preliminary crash report is expected in two to three weeks and a full investigation could take up to two years, officials said.
The small plane departed from Renton Municipal Airport around 9:30 a.m. and at about 10:20 a.m. crashed and then burned in a field east of Harvey Airfield in Snohomish County, the NTSB said.
The names of the people on board haven’t been released. Raisbeck Engineering of Seattle in an emailed statement over the weekend identified them as two "highly experienced" test pilots, a flight-test director and an instrumentation engineer.
The crew was flying the plane in preparation for modifying the aircraft, Raisbeck President Hal Chrisman said in the statement.
NTSB investigators are collecting plane maintenance records and information about the pilots’ licenses, ratings and history, officials said.
The Snohomish County medical examiner has not yet identified the victims. Chrisman declined to further comment to the newspaper on the people involved in the crash.