Recent changes to a control system and rapid speed changes made by an operator led a Texas mother and her two young girls to be thrown about 25 feet off a Ski Granby Ranch chairlift in December 2016, state investigators said in a final report on the incident.

The 40-year-old woman — Kelly Huber — was killed and her two children hurt in what the report said was an “unprecedented” sequence of events that none of those who reviewed the case had ever seen before.

“No one on the investigative team has ever witnessed or heard of a similar event,” said the 151-page report, released Thursday afternoon. “Likewise, literature does not describe such an event.”

The Colorado Passenger Tramway Safety Board report, released more than four months after the fatal fall in Grand County, squarely places blame for the incident on a lift malfunction and ends speculation about an incident that gripped the ski industry. Huber’s death was the first related to a lift malfunction in the U.S. since 1993 and the first fatality from a chairlift fall in Colorado since 2002.

Huber was pronounced dead the day of the fall — Dec. 29, 2016 — at Middle Park Medical Center in Granby, where she was taken about an hour after being ejected from her chair when it slammed into a lift tower. The drive control system at the center of the incident, typically housed in the operator’s shack and which handles a lift’s power and operator inputs, had been installed less than a month before.

Witnesses told state investigators that the chair Huber and her daughters were riding on — chair 58 — began to violently sway as it approached lift tower five, according to the final report. When it got to the tower, the two collided.

A Ski Granby Ranch ski patroller wrote in an incident report that he was on a chair just in front of Huber and her girls. He said he heard a rumble on the lift’s line and that he felt the “largest vertical motion I had ever felt in a lift line” as he grabbed the side of his chair.

The patroller looked back to see Huber and the girls falling to the ground. “The mother was visibly holding one of the children in what appeared to be an attempt to protect the child from the impact of landing,” he wrote.

Another witness to the fall, a resort guest, wrote in a witnesses statement: “Don’t let her die.” A third person wrote: “Please let me know if they’re OK.”

The report also says witnesses reported the lift made several sudden accelerations and decelerations in the moments before Huber and her children fell from the lift. State investigators said the speed changes were a secondary cause of the fatal fall. The drive control system was listed as the main culprit and investigators said that it possibly created pulses of energy along the rope line and “could explain the rope instability.”

“Until the new drive was installed in December 2016, there had been no major electrical changes since the lift’s construction,” the report said.

The report added: “It is the conclusion of the investigation team that the selected tuning of the drive combined with the natural harmonics of the lift system, along with rapid speed changes, caused the rope instability resulting in (chair) 58 contacting tower five.”

The Grand County coroner ruled Huber died of blunt force trauma to her torso and a traumatic rupture of her aorta. The four-person, high-speed Quickdraw Express lift, built by Grand Junction-based Leitner-Poma in 1999, was closed after Huber’s death.

Ski Granby Ranch said work by an independent contractor on the lift’s electrical drive control system before the start of the ski season likely led to the conditions that caused Huber’s death. The resort said the contractor is not affiliated with Leitner-Poma. Also, the resort said it “had followed all prescribed protocols in operating the lift.”

Ski Granby Ranch CEO Melissa Cipriani said in a news release that the Quickdraw Express lift was load-tested Dec. 5, before the ski season began Dec. 16. She said the Huber family’s fall was the first incident of its kind in the resort’s 22 years of operation and that the lift has been operating safely since its installation.

On Thursday, Granby Ranch officials said they were reviewing the report. “This is a 151-page report that deserves careful review. Granby Ranch continues to comply with all Tramway Board directives. We would again like to offer our condolences to the Huber family for their loss. Granby Ranch is committed to the health and safety of its guests.”

The Colorado Passenger Tramway Safety Board declined to make available a representative for comment.

The report says that because of the unprecedented nature of the incident, investigation and analysis of what led up to Huber’s death will “likely continue for years.”

State investigators also found seven “potential contributing factors” to the fatal fall, including other equipment replacements and damage to a motor system.

Investigators made 10 recommendations in light of their findings, mainly that chairlifts undergo more detailed testing to include different simulated load parameters. They also suggested delays are needed between when a chairlift’s speed levels are changed and between the time a lift is stopped and restarted.

Finally, investigators also urged installation of a “black box” on all chairlifts that can record stops, starts and speed changes.

Of the more than 250 chairlift falls in Colorado since 2002, most have been attributed to skier error, according to state data.

According to an October report by the National Ski Areas Association, the last death on a U.S. chairlift attributed to a malfunction was in 1993. The trade group also said from 1973 to October, there were just 12 deaths attributed to chairlift malfunctions.

During that 43-year span, the NSAA says, an estimated 16.7 billion lift rides were taken by skiers and snowboarders. As of the 2015-16 ski season, the annual fatality rate per 100 million miles traveled on ski lifts was 0.14.

In 1976, two cars from Vail’s 7-year-old gondola — each carrying six skiers — plummeted 125 feet, killing four people in one of the most deadly lift incidents in the United States. In 1985, a bullwheel at Keystone Resort failed, sending waves down the line that threw 60 people off the Teller Lift, two of whom later died of their injuries.

The ski industry has stressed since Huber’s death that chairlift falls — especially fatal ones — are extremely rare considering the millions of rides taken each year.

Huber’s then-9-year-old daughter was taken to Middle Park before being flown by Flight for Life to Children’s Hospital Colorado after the fall.

Her other daughter, who was 12, was treated at Middle Park and released.

Huber, who lived in San Antonio, was vacationing in Colorado with her daughters and other family members at the time of the fall, authorities say.

The investigation into Huber’s death has been forwarded to the Colorado Department of Regulatory Affairs investigations office for further review. Once that probe is complete, the tramway safety board will determine if there were any violations of the Colorado Passenger Tramway Safety Act.

It wasn’t clear Thursday when that process would be complete.