TUKWILA, Wash. -- An 11-year-old autistic boy who was struck by a vehicle as he was attempting to cross the northbound lanes of SR-99 just north of SR-599 in Tukwila Saturday evening has died.

The Washington State Patrol tells KIRO 7 that the boy passed away at Harborview Medical Center Sunday afternoon.

According to Trooper Dan McDonald, of the Washington State Patrol, at 4:45 p.m. Saturday, WSP received reports of a juvenile male walking on the left shoulder of the northbound lanes of SR-99. Less than 1 minute later WSP received additional 911 calls reporting that the boy had been hit.

At 4:29 p.m., the autistic boy's aunt had reported him missing from his SeaTac residence. As family members, neighbors, and King County Sheriff's deputies were frantically searching the area, the boy had wandered toward the freeway, crossed the southbound lanes, and was attempting to cross the northbound lanes when he was hit by a 1-ton flatbed pickup.

McDonald tells KIRO TV that the driver of the truck, a 46-year-old Seatac resident, was not injured.

Alcohol and/or drugs are not believed to have been a factor in the crash, and no charges are pending at this time.

The boy was treated by police and aid personnel at the scene. He was transported to Harborview Medical Center in critical condition.

The right northbound lane of SR-99 was closed for just under three hours while troopers investigated the scene.