The National Transportation Safety Board agency released 1,400 pages of documents about the crash, USA Today reported. They include statements from another pilot with the sightseeing company Sundance Helicopters that he heard a scream over the radio just before the crash and from witnesses that the engine was "screaming or red-lining."
The NTSB said there has been no determination yet of the cause of the Dec. 7 crash, which occurred between Las Vegas and Hoover Dam.
The crash killed Landon Nield, 31, the pilot, and two couples. Lovish Bhanot, 28, and Anupama Bhola, 26, had come to Las Vegas from India for their honeymoon, while Delwin and Tamara Chapman, both 49, from Kansas were on a 25th-anniversary trip.
The pilot who testified about the scream said Nield's helicopter had just had an overhaul, with the engine and two valves replaced. He said he heard the scream on his radio.
"It sounded like a scream from the pilot and not that of passengers in the background," he said.
Sundance operates helicopter tours around Las Vegas and the Grand Canyon. The company had a fatal crash in 2003 near the Grand Canyon.