A fourth person has died as a result of injuries sustained during Wednesday's fog-blanketed pileup on I-215.

Dead is:William A. Boes, 49, Salt Lake City.

Boes died at 3:30 p.m. Friday of severe head injuries at LDS Hospital, said Craig Rasmussen, hospital spokesman.

The other three people died Wednesday.

The accidents occured in the eastbound lanes between Redwood Road and Davis County between 9:45 and 10 a.m. Wednesday and involved about 50 vehicles.

Investigation into the causes of the accidents is ongoing, but officials believe that negligence will be hard to prove.

"This accident is going to be a nightmare to reconstruct," Davis County Attorney Mel Wilson told the Associated Press. "I would doubt that they will come up with the evidence that will allow us to prove negligence unless alcohol was involved."