A gondola at Whistler resort that collapsed, injuring a dozen people, went down because water seeped in and then ripped it apart when it froze, says a report by the British Columbia Safety Authority.

The authority has issued a safety order for all ski hill towers in the province to be fitted with drain holes by November to prevent any similar incidents.

Six gondola cars were damaged when they fell from the Excalibur line at Whistler Blackcomb in December 2008.

In photos: Blackcomb Gondola Incident

Forty-three passengers were riding the lift at the time and most walked away with bumps and bruises, though one person suffered a fractured vertebra.

In photos: Blackcomb Gondola, Day Two

The safety authority's report said the tower was designed to prevent water from seeping in -- yet it did, likely through a plate connecting the tower's upper and lower sections.

The water then froze and the ice expanded inside the cavity, pushing against the walls and the plate, a phenomenon known as "ice-jacking," and triggering the collapse.

"The result of the 18-month investigation is the primary cause was the intrusion of water, and ice being allowed to form," Jason Gill, the authority's manager for passenger ropeways, said in an interview.

The authority, which oversees the province's gondolas, ski lifts, trams and other ropeways, made six recommendations in its report.

Chief among them was ordering all B.C. ski hills to ensure towers have drain holes by Nov. 1.

Though a number of lawsuits were filed against Whistler Blackcomb following the crash, the safety authority's report stopped short of laying any blame.

"Our intention with this report is not to lay blame, it is to identify the root cause and to make sure that we put recommendations in place to prevent a reoccurrence," Gill said.

At the time of the collapse, the Excalibur gondola had been in service for 14 years with no serious incidents reported.

Doug Forseth, senior vice-president of operations for Whistler Blackcomb, welcomed the report.

"We've been working closely with BC Safety Authority since the incident occurred ... so we've had a sense of the direction this was going. It's nice to have (the report) finished and out," he said in an interview.

Forseth said Whistler Blackcomb has already put drain holes in its several hundred towers.

"We've learned that there's an improvement that can be made so I think that's a good thing and hopefully that will resolve issues again before they happen in the future."

The report said although tests for water accumulation were conducted during periodic inspections of the tower, resort staff didn't detect any because the test method is dependent on the tester's knowledge of the tower construction.

It recommended manufacturers clearly communicate how to undertake an effective inspection, and called on passenger ropeway contractors to ensure safety bulletins are followed correctly. The safety authority also recommended improving its own communication process.

The report noted that the record of inspections carried out on the tower was quite vague.

Other factors, such as low temperature toughness, a pre-existing crack and stresses from ferrying skiers up the mountainside may have also played a role in the accident, the report said.

Dozens of passengers were stranded for hours after the collapse on Blackcomb Mountain, which neighbours Whistler Mountain, the home of alpine events at the 2010 Winter Games.

One gondola car hung over a creek as resort and emergency officials worked to stabilize the tower before they could attempt an evacuation.

The incident on the Excalibur was not the first ski lift accident at the resort.

A Dec. 23, 1995, accident on the Quicksilver lift killed two men and injured nine other people.

The high-speed lift was ferrying skiers to the top of a run when one chair slipped on a cable and slammed into another, setting off a cascade that sent four chairs crashing into the bush and rocks three storeys below.

A coroner's report said the accident was a result of systemic failure, noting problems with the lift system's grip mechanisms should have been detected in advance.

In January 2006, two empty gondola cars at the Sunshine Village ski resort near Banff, Alta., plummeted to the ground after being dislodged from their cables by high winds. No one was injured.