What caused Seattle crane to collapse?

But the possibility that tower bolts or pins were removed early, while the crane was being dismantled in gusty but not exceptional winds, is sure to be a focal point for investigators. But Kwass, who is not representing any parties involved with last month’s collapse, also thinks that the South Lake Union crane had its pins pulled, or at least loosened, all the way down the tower, ahead of time, like the one near Dallas. The UK’s Health and Safety Executive, a government agency, ultimately found that collapse, which killed two workers, was “due to mast section bolts being untightened and left ‘finger tight’ in preparation for dismantling the crane.”

Source: www.seattletimes.com