4 People Killed in Calif. Helicopter Crash: 'Mind-Boggling,' Says Witness

Four people were killed in a helicopter crash on Sunday in a rural area of northern California, officials said.

Authorities in Colusa, about 70 miles northwest of Sacramento, received a report of a helicopter crash near Highway 45 around 1:15 p.m., the Colusa County Sheriff's Office said in a press release.

When first responders arrived, they found four passengers on board, all of whom were pronounced dead at the scene.

Those four people were the only passengers on board the plane, a Robinson R66 helicopter, a Federal Aviation Administration (FAA) spokesperson says in a statement to PEOPLE.

"The FAA will release the aircraft tail number when investigators verify it at the scene. The FAA and National Transportation Safety Board will investigate," the statement reads. "Neither agency identifies people involved in aircraft accidents."

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The sheriff's office said that an official cause of death has not yet been determined, and that the passengers' identities will not be released until proper notification has been made.

Farmer Walt Seaver told KOVR that while he does not know the victims' identities, he does know the owner of the helicopter.

"Nice man, very nice man. I'm in shock, basically, it's a small community. I consider their family a pillar of the community," he said, adding that he is "not quite giving up yet" on hopes that the man, a former classmate of his, did not die in the crash.

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Seaver said he was working on his ranch across the street when the plane went down, but was surprised by how quiet the crash was.

"Kind of mind-boggling — didn't hear anything, didn't see anything," he told KOVR. "From what I understand, once it went down, I couldn't render aid to anybody anyway."

The sheriff's office is still investigating the scene and is interviewing people who may have seen the accident unfold, though the primary investigation is being handled by the FAA and that National Transportation Safety Board (NTSB), the release said.

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Jennifer Gabris, an NTSB spokesperson, told CNN that the investigation will include requesting radar data, weather information, air traffic control communication, airplane maintenance records and pilot medical records.

She said a preliminary report will be available after 15 days, while completion of the full investigation will take between one and two years.