Tons of sand were putting stress on the Interstate 35W bridge in Minneapolis at the time it collapsed last August, according to the National Transportation Safety Board.

The New York Times reported Tuesday that the NTSB, which is still in the process of reconstructing the circumstances of the collapse, released a diagram showing the location of every car, truck and piece of construction equipment on the bridge at the time of the collapse. It shows four mounds of sand weighing an estimated 99 tons placed on the bridge by construction workers.
"In all, board researchers calculated a load of 1.26 million pounds, or 630 tons, including 198,920 pounds of sand at the critical spots," says the New York Times report. "However, the load would not have been excessive for a well-designed bridge."
The I-35W bridge collapsed Aug. 1, killing 13 people and injuring 145. The NTSB has not yet established the cause of the collapse, but says it will do so by the end of the year.