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(CNN) —  

It may be some time before we know for sure what caused a construction crane to come crashing down on a Seattle street, killing four people – but some experts say human error could have played a role.

On Saturday, a crew was disassembling a crane on the roof of a building that’s being constructed as part of the new Google Seattle campus. The mast toppled, killing two bystanders and two ironworkers.

“I think there’s a 99% chance that this is a human error cause and not a structural or mechanical failure by the machine,” said Timothy Galarnyk, a construction safety expert.

Five companies are now under investigation, and authorities are still working to discover exactly what caused the crane to collapse.

It may take up to six months to complete the investigation, said Tim Church, a spokesman for the state Department of Labor & Industries. “This allows us access to interview workers, access company records and perform other investigative work,” he said.

Were pins removed too soon?

Once a construction company finishes using a crane, a team will spend between two or three days taking it apart, piece by piece.

After reviewing video and photos from the scene, David L. Kwass, a trial attorney who has handled crane accident lawsuits, said it looked like the pins connecting the crane’s segments could have been removed prematurely, facilitating the fall.

“It’s possible that they (ironworkers) went all the way down to the base of the crane and they popped out all the pins,” said Kwass, who is with one of the few law firms who investigate these types of accidents. “They are not supposed to pop out those pins until all the sections are under control.”

“It has to go step by step. You go ahead and these accidents happen.”

If portions of the crane were unsecured and unstable, the weight shift, along with the wind gusts between 18 and 23 mph that were reported in the area, could have contributed to the collapse, Kwass said.

Did workers follow the rules?

Galarnyk, president of the Minnesota consulting firm Construction Risk Management Inc., believes that the workers likely did not follow the manufacturer’s instructions for dismantling the crane.

“Construction is not dangerous, it’s hazardous. When people don’t follow the rules, it becomes dangerous,” he said.

One person who is fully familiar with the manufacturer’s instructions usually oversees the assembly and disassembly, Galarnyk said.

“When the A/D director is not managing an assembly rigidly, you are going to have an accident,” Galarnyk said.

“If workers don’t comply with the manufacturer’s instructions, it’s not a matter of if this will fail, it’s a matter of when,” he said.

While the industry is closely regulated for crane operators, those responsible for assembling or disassembling cranes don’t have similar oversight.

Craig Hautamaki, director of operations for the Colorado Crane Operator School, said certification would help, but ultimately it’s the company’s responsibility to make sure people know what they’re doing.

A similar incident reported in Texas

While crane collapses are uncommon, experts said the Seattle incident was similar to a collapse that took place at The University of Texas at Dallas in 2012. A construction crane was being dismantled when it fell across a building, killing two workers, officials said.

A report issued by the Occupational Safety and Health Administration indicates that workers either loosened or removed bolts to save time during dismantling.

From 2011 to 2015, an average of 44 people were killed every year in accidents involving cranes, according to the Bureau of Labor Statistics.

In 2016, a crane came down in lower Manhattan, crushing several parked cars and killing one person inside. The investigation found the operator failed to properly secure the rig. In 2017, three cranes designed to withstand 95 mph winds in south Florida were outmatched by the force of Hurricane Irma, causing them to partially collapse.

In Washington, then-Gov. Christine Gregoire signed a law that required the state’s Department of Labor and Industries to set up a certification program for cranes after a tower crane collapsed in 2006 and killed a Microsoft attorney who was inside a building in downtown Bellevue.

CNN’s Scott McLean contributed to this report.