WASHINGTON, D.C.—The National Transportation Safety Board (NTSB) revealed in mid-January that undersized gusset plates were found at eight of the 112 nodes (joints) on the main trusses of the I-35W bridge that collapsed Aug. 1, 2007. These 16 gusset plates (two at each node) were roughly half the thickness required and too thin to provide the margin of safety expected in a properly designed bridge, the NTSB said.

The probable cause of the accident will be determined when the final report, expected before the end of the year, is presented to the NTSB.