Witnesses saw pieces of a sightseeing helicopter fall from the aircraft moments before it crashed on Molokai in 2011, killing all five aboard.

The accounts were included in documents released late Tuesday by the National Transportation Safety Board.

"Several witnesses reported that their attention was initially drawn to the helicopter when they heard what they described as a ‘woop wooping’ sound," the report said. "Witnesses also consistently reported that as the helicopter continued its steep, southerly descent, fragments of wreckage were observed falling from the helicopter."

Neither this report, nor others issued in April and May, reached any conclusion as to what caused the Blue Hawaiian Helicopters tour aircraft to crash on a slope above Pukoo on Nov. 10, 2011.

"The investigation is just ongoing," David Chevalier, president of Blue Hawaiian Helicopters, said by telephone Wednesday.

Killed in the crash were tour pilot Nathan Cline, 30, of Kihei; honeymooners Michael Todd Abel, 25, and Nicole Bevilacqua-Abel, 28, of Pittsburgh; and businessman Stuart Robertson, 50, and his companion, Eva Birgitta Wannersjo, 47, an accounting assistant, both of Toronto.

Witnesses told investigators the crash happened right after an unusually strong squall.

Jay Duquette, an off-duty firefighter, said he looked up when he heard a "very loud hissing sound followed by a ‘woop, woop, woop’ sound," the report recounted. He also saw what could have been a "wispy trail of smoke" behind the chopper before it crashed in a fireball.

Floyd Kapuni said he heard a "weird tat, tat, tat noise" and looked up to see the back of the helicopter’s engine glow cherry-red like the back of a missile.

Walter Paleka said he saw pieces of the aircraft fall off during its descent, then the main rotor blade "stopped and popped" off the helicopter. While it was still airborne, the helicopter’s cabin was "on fire, no explosion, just fire," he said.

The copter went straight down without spinning, he added.

Cline was on his third flight of the day, departing from Kahului Airport with four passengers for a round-trip sightseeing flight to Molokai. He reported his departure from the Kahului airport at 11:44 a.m., and the crash occurred at 12:14 p.m.

Another tour pilot told investigators that weather conditions on Molokai were continually deteriorating with a strong northeasterly wind and fast-moving squalls.

The report includes statements from six witnesses on the ground and from pilots of other tour companies operating around Molokai at the time.

"All witnesses reported that around the time of the accident, heavy localized rain showers with strong gusty wind conditions were in the area," the report said.

The report said Cline had recently moved from Southern California to Maui and his previous helicopter flight experience was gained as a contract pilot flying Bell helicopters for Bristow International Helicopters in the Gulf of Mexico.

At the time he was hired on July 1, Cline had no flight time in Blue Hawaiian Helicopters’ EC-130B4.

Cline had accrued 306 flight hours in an EC-130B4 helicopter while employed at Blue Hawaiian and had passed a federal competency air ride the day before the crash, the report said.