A helicopter carrying two Virginia state troopers, shook and then began to spin before it disappeared from view, witnesses told federal investigators looking into the cause of the crash that killed the two officers during last month’s white nationalists’ rally in Charlottesville.

According to a preliminary report released Tuesday by the National Transportation Safety Board, Lt. H. Jay Cullen and Trooper-Pilot Berke M.M. Bates had just completed surveillance over downtown Charlottesville and were on their way to provide air support for the motorcade of Virginia Gov. Terry McAuliffe (D) when their helicopter disappeared from radar just before 5 p.m.

Witnesses said they saw the helicopter hovering before it began a “rolling oscillation” and began to spin. The aircraft then “descended in a 40-degree nose down attitude,” before it disappeared from view the report said. Security camera footage corroborated those accounts. Witnesses then reported seeing a plume of smoke from the crash site. An earlier NTSB report said there was no distress call prior to the crash.

While the preliminary report did not include a cause, investigators said there was no indication that the Aug. 12 crash was the result of a mid-air collision with another aircraft, animal or object. The helicopter left a debris field several hundred feet long to the west of the main wreckage. Some debris was found on the roof of a residence adjacent to where the helicopter came to rest, said investigators who interviewed more than three dozen witnesses. While all the main helicopter components were recovered at the site of the crash, most were damaged in the crash and during the post-crash fire, investigators said.

The Bell 407 helicopter had been in operation since 2000, according to aircraft maintenance records and had undergone its 100-hour inspection less than two weeks before the crash. While not noted in Tuesday’s preliminary report, the aircraft had previously crashed in 2010. No one was injured in that incident, but NTSB investigators said the probable cause was faulty maintenance by a shop that was not authorized to make repairs on that particular aircraft. An NTSB spokesman said it it not clear whether there is any connection between that crash and the one last month, but that it will be considered as part of the broader investigation.

“The Virginia State Police appreciates the professionalism and comprehensive investigative efforts of the NTSB during all aspects of this incident,” said spokeswoman Corinne Geller.

Both officers had flight experience and were  particularly close to the McAuliffe family. Cullen, 48, was commander of the state police aviation unit, had spent several years shepherding the governor around Virginia.

At his Cullen’s funeral, McAuliffe said he considered the trooper part of his family.

“Dorothy and I are heartbroken,” McAuliffe said. “It’ll never be the same when I step into that helicopter and not see Jay in that front right seat with Cullen on the back of his helmet.”

Bates, 40, served on the governor’s security detail and had only recently transferred to the aviation unit. The day before he died, Bates had contacted the governor about sending a care package to the McAuliffe’s oldest son, a Marine recently deployed to the Middle East.

The full investigation into the cause of the crash is expected to take 12 to 18 months.