According to documents released by National Transportation Safety Board investigators Monday, the construction materials piled on a narrow, 115-foot section of roadway on the I-35W bridge over the Mississippi River comprised more than half of the weight on the main span when it tumbled into the water last Aug. 1, the St. Paul Pioneer Press reported Tuesday. The bridge span's connecting plate was half the thickness it should have been, the newspaper reported.

In addition to the piles of aggregate, which were part of a road resurfacing project, there were nearly 100 tons of cement mixers, water trucks and other construction equipment nearby, the Pioneer Press said.

The board, which reported in January it was focusing on a flaw in the design of the bridge's gusset plates, is expected to complete its investigation into the accident by the end of the year.

"Significant progress continues to be made in the investigation," NTSB Chairman Mark Rosenker said in a statement.

In addition to those killed, 145 people were injured.