A buckled steel plate may be all it takes to bring down a 40-year-old structure like the Minneapolis I-35W bridge, which collapsed suddenly on Aug. 1 and killed 13 people.

Forensic engineers probing the collapse have not yet found all the bridge's gusset plates but have seen enough damage "in some gusset plate locations that warrants further investigation," the National Transportation Safety Board says in a Sept. 5 bulletin.

NTSB already has raised questions about design flaws in the bridge's gusset plates.

The 1,907-ft-long, eight-lane bridge crossed the Mississippi River in three spans on two 988-ft-long, parallel deck trusses. They connected to transverse-mounted floor trusses supporting the roadway. Dozens of flat gusset plates reinforced the steel joints where hundreds of vertical, horizontal, diagonal and cross beams intersected.

Such deck-truss designs are "now considered obsolete," NTSB continues. A failure in one of the plates "could have catastrophic consequences" due to the structure's "inherent lack of redundancy." The vintage design led state inspectors to slap a "structurally deficient" rating on the bridge in 1990.

Whether or not gusset plates buckled under the 287 tons of construction equipment and materials staged on the bridge on Aug. 1 is being investigated.

"Much of the bridge superstructure is still under water," says NTSB Chairman Mark V. Rosenker. "There is still considerable work remaining to determine why it collapsed."

Crews were in the middle of concrete deck repairs when the bridge tumbled into the water below. NTSB says it will use a "very detailed" finite-element computer model to determine loading on each bridge component prior to the collapse.

The probe is expected to take a year to 18 months.