An iron cross bearing the numbers of Ironworkers Local union chapters from Portland and Seattle stands next to flowers, Monday, April 29, 2019, at a memorial on a median across the street from where a construction crane collapsed and fell Saturday, killing four people, two of whom were ironworkers. (AP Photo/Ted S. Warren)

SEATTLE (AP) — It appears workers who were disassembling a construction crane had prematurely removed pins securing the sections of the crane's mast to each other, and that could explain why it toppled Saturday, killing four people, experts said.

Crane accident investigator Tom Barth said Monday based on videos of the Seattle crane collapse that "if the pins had been in, that crane would not have fallen over."

The crane's mast fell over on Saturday afternoon as workers were taking it apart. Sections landed on top of the new Google building where it had been used and on traffic below, striking six vehicles. Two ironworkers on the crane were killed, as were two people in cars — a retired city employee and a college freshman. Four others were injured.

"The reason this tower fell over is ironworkers and the people working on it did not follow the manufacturer's instructions for disassembling the crane," Barth said.

Barth, who has been a tower crane operator, inspector and accident investigator for 38 years, said the only safe way to disassemble a tower crane is to do it by sections.

First, a line from another crane is secured to the top of the section being removed. Only then do workers remove the bolts that secure the section being removed to the one below it. The second crane lowers the section, Barth said.

The videos, including one from a dash camera in a car approaching as the crane fell, appear to show the sections of the tower crane separating cleanly, indicating the pins had likely been removed early, said Dave Kwass, a trial attorney who handles crane accident lawsuits.

"Where I would be starting as an investigator is getting a better grasp of what these two ironworkers who tragically were killed may have done in removing the pins," Kwass said. "What I'm seeing is a crane that looks as though the pins were popped all the way down to the pedestal section."

Kwass also suggested that the shifting weight of the workers at the top of the mast as they tried to grab the wind-swayed hook and line from the other crane might have helped push the mast over.

Early pin removal was blamed after a tower crane mast collapsed during disassembly in Dallas in 2012, killing two workers. Terry McGettigan, a tower crane specialist in Seattle, declined to comment on Saturday's crash, but he wrote a paper on the Dallas collapse warning about the danger of taking shortcuts by loosening or removing the bolts in advance.

The Iron Workers Union Local 29 based in Portland, Oregon, and Local 86, in Seattle, represented the workers who died and did not respond to requests for comment Monday.

Washington Department of Labor and Industries inspectors finished their work at the scene Monday, said Tim Church, a department spokesman. He said it was too early to speculate on a cause.

The department on Monday expanded its investigation to include a fifth company: Seaburg Construction Corp., which employed the tower crane operator. The other companies are general contractor GLY, Northwest Tower Crane Service Inc., Omega Rigging and Machinery Moving Inc., and Morrow Equipment Co. LLC

"GLY and its sub-contractors involved with this tower crane accident are doing everything we can to investigate the incident," the company said in a written statement. "We are cooperating fully with investigators and assisting the local authorities. At this early stage of the investigation, we have no further details."

The ironworkers who died were identified by their unions as Andrew Yoder, 31, of Washington, and Travis Corbet, 33, of Oregon. Corbet's wife, Samantha Corbet, told Seattle television stations that her husband had served in the U.S. Marine Corps for five years before becoming an ironworker.

They married last year and planned to go on their honeymoon in June.

Also killed were Seattle Pacific University freshman Sarah Wong, 19, and Alan Justad, 71, who retired in 2014 as deputy director of Seattle's Department of Planning and Development.

In a written statement, Mayor Jenny Durkan called Justad "a true public servant."

An iron cross bearing the numbers of Ironworkers Local union chapters from Portland and Seattle stands next to flowers, Monday, April 29, 2019, in Seattle at a memorial on a median across the street from where a construction crane collapsed and fell Saturday, killing four people, two of whom were ironworkers. (AP Photo/Ted S. Warren)
Damage to a building that was under construction when a crane collapsed on it Saturday in Seattle is shown, Monday, April 29, 2019, in Seattle. The accident killed four people. (AP Photo/Ted S. Warren)
Fire and police crew members work to clear the scene where a construction crane fell from a building on Google's new Seattle campus crashing down onto one of the city's busiest streets and killing multiple people on Saturday, April 27, 2019. (AP Photo/Frank Kuin)
With a portion of the broken crane on the roof behind, a worker suspended in a basket clears debris from a building damaged when the crane atop it collapsed a day earlier, Sunday, April 28, 2019, in Seattle. The construction crane fell from a building on Google's new campus during a storm that brought wind gusts, crashing down onto one of the city's busiest streets and killing multiple people. (AP Photo/Elaine Thompson)
Emergency crews work at the scene of a construction crane collapse where several people were killed and others were injured Saturday, April 27, 2019, in the South Lake Union neighborhood of Seattle. The crane collapsed near the intersection of Mercer Street and Fairview Avenue pinning cars underneath it near Interstate 5 shortly after 3 p.m. (AP Photo/Joe Nicholson)
A construction crane working on a building collapsed near the intersection of Mercer Street and Fairview Avenue near Interstate 5, Saturday, April 27, 2019, in downtown Seattle. Authorities say several people have died and a few others are hospitalized after the construction crane fell onto a street pinning cars underneath Saturday afternoon. (Genna Martin/seattlepi.com via AP)
Emergency crews work the scene of a construction crane collapse near the intersection of Mercer Street and Fairview Avenue near Interstate 5 in Seattle, on Saturday, April 27, 2019. Authorities say several people have died and a few others are hospitalized after the construction crane fell onto a street in downtown Seattle Saturday afternoon. (Joshua Bessex/The News Tribune via AP)
Emergency crews work the scene of a construction crane collapse near the intersection of Mercer Street and Fairview Avenue near Interstate 5 in Seattle, on Saturday, April 27, 2019. The crane was atop an office building under construction in a densely populated area. Authorities say several people have died and a few others are hospitalized after the construction crane fell onto a street in downtown Seattle pinning cars underneath on Saturday afternoon.  (Joshua Bessex/The News Tribune via AP)