The designer of the concrete ceiling panels in Boston’s Big Dig tunnels originally wanted to use four bolts on each anchor plate.

But, the Boston Globe has obtained a memo showing that project managers from Bechtel/Parsons Brinkerhoff, the firm behind much of the massive construction projects, convinced the designer to use only two bolts instead.

The Globe said those bolts were then safety-tested at a much lower standard than engineers typically required at the time.

Twenty of those same bolts came loose July 10, causing several ceiling panels to collapse on a car, killing its passenger.