MINNEAPOLIS --- Federal safety investigators have concluded that the original designers of the Interstate 35W bridge apparently neglected to calculate the required size of key gusset plates, a mistake that 40 years later resulted in the death of 13 people after the span collapsed, the Star Tribune reported.

Investigators also have found that corrosion of certain gusset plates, extreme heat and shifting piers did not contribute to the bridge's collapse on Aug. 1, 2007, sources told the newspaper.

Investigators will present their findings to the National Transportation Safety Board (NTSB), which will publicly review the draft report in a hearing Nov. 13 in Washington. After that, the board will use the draft as the basis for its final report on the probable cause of the bridge collapse and recommendations for preventing future disasters.

If key steel gusset plates had been designed properly -- they were one-half inch thick instead of an inch -- the bridge would have been able to withstand tons of concrete and steel added in two renovation projects as well as the 287-ton construction load on the bridge the day it collapsed, sources told the Star Tribune.

In January, NTSB Chairman Mark Rosenker was criticized by U.S. Rep. Jim Oberstar (D-MN.) and others for placing too much early emphasis on gusset plate failure as the potential cause. But the investigators' findings appear to validate Rosenker's early statements.

Investigators found records showing that the bridge designers knew how to calculate the thickness of gusset plates, but probably did not perform that task for the bridge's center portion, where the initial failure occurred, the Star Tribune reported. Investigators leave open the possibility that the design firm, Sverdrup & Parcel, carried out the calculations but erred. That assessment is based on a review of the original work papers of the St. Louis-based firm. The firm was later acquired by Jacobs Engineering of San Francisco.