NEW ORLEANS — Investigators now know what may have caused the wheel assembly of a tractor trailer to come off and careen into a Gautier, Mississippi rest stop on March 5. 

The wheels hit Tulane University student Margaret Maurer, killing her.

A Mississippi Department of Transportation official told the New Orleans Advocate the absence of a locking washer, a part that costs $3, was cited as a probable cause of the incident.

The 3-inch metal ring was missing when inspectors unpacked the outer hub of the wheel assembly following the fatal accident, the official said.

RELATED: Missing $3 part blamed for Tulane student's death by runaway big-rig tires

Tommy Landa, owner of SOS Truck Repair in Chalmette, said locking washers are a crucial piece needed to hold wheels in place.

"It holds the bearings in place," Landa said. "It makes it one complete unit. It is very important. Without it, the wheels will come off."

Police officials said two wheels joined together, crossed the interstate and traveled nearly 300 yards before striking Maurer as she and friends were returning to their car in an I-10 rest stop.

"However fast the vehicle is going, that's how fast the tires are coming off," Landa said. "I've seen them run a mile and a half before we could catch them."

RELATED: Tulane student killed by flying tires at highway rest stop

Landa added the fatal accident underscores the importance of making sure that everything that leaves his shop is fixed correctly and is 100 percent before it gets back out on the road.

"Everything we do now has the potential of affecting somebody's life," Landa said.

The MDOT's findings are preliminary, officials said. 

RELATED: Car flips on I-10 east, killing driver

Stay with the Eyewitness News on WWL-TV and WWLTV.com for more on this developing story.