State safety inspectors are examining the possibility of missing connector pins as the primary cause of a construction crane accident in downtown Seattle that left four people dead Saturday afternoon, sources close to the investigation said Monday.

The 3:30 p.m. accident near the corner of Fairview Avenue and Mercer Street occurred during gusty winds as the giant crane was being disassembled. Witnesses said multiple sections of the structure came loose and plunged to the street, crushing several cars. Two people in cars died along with two ironworkers who were working on the crane.

State Labor and Industries inspectors are focused on sequence of events leading to the accident, specifically: when were the crane arm connector pins removed, and when was the disassembly crane attached? According to the engineers and crews who assemble large building cranes seen all over Seattle, an assembly crane is used for the build up and eventual take down of large construction cranes.

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But on the Mercer accident at the Google building project no pins — broken, sheared or otherwise — were found on the arms or in the debris field by people on scene afterwards. This has led inspectors to speculate that the pins might have been removed prior to the disassembly crane being attached as a backup.

Coupled with the sudden and unexpected wind gusts, the sections of structure might have been loose and left vulnerable to collapsing, sources said, because they were not already anchored to the take-down crane.

“It doesn’t look good,” said one person who was at the scene and asked to remain anonymous.

But many questions remain. First, if the pins were removed prior to attachment of the disassembly crane, did the ironworkers detach it under the instruction of a site engineer or supervisor? And, if so, why wasn’t standard procedure followed? As one person familiar with the accident aftermath put it, “this wasn’t the decision of a single rogue ironworker. ”

The Seattle Times reported that pin or bolt removal also was to blame for a similar 2012 incident the Dallas campus of the University to Texas. According to the Times: “That tower crane collapse, which killed two workers at the University of Texas at Dallas, also occurred during dismantling and featured just the crane’s mast, not its horizontal arm.

“Prior to dismantling of the mast, the employer removed mast bolts from every other tower section,” an investigation by the federal Occupational Safety and Health Administration (OSHA) found. “Some of the remaining bolts were also de-torqued.”

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