All four people aboard a single-engine plane were killed when it crashed Friday morning just north of Seattle, Washington, officials said Saturday.

The single-engine Textron 208B went down at 9:35 a.m. local time Friday in a field east of Harvey Field Airport in Snohomish County, according to the Federal Aviation Administration.

The FAA had initially indicated Friday that two people were aboard the plane. However, the Snohomish County Sheriff’s Office confirmed to CBS News on Saturday that there were four people aboard, all of whom died.

The identities of the four victims have not been released.

The crash scene was in an “unworked field,” Snohomish County Fire District 4 said, and crews had difficulty accessing it due to the “terrain, vegetation and irrigation canals.” Firefighters struggled to extinguish a large fire caused by the wreck, and had to lay down 2,000 feet of water hose to battle the flames, the fire department said.

The FAA and the National Transportation Safety Board are investigating the cause of the crash.