Thomas Timko, 41, who was shot in the head, remained at the Hospital of the University of Pennsylvania in serious condition, an improvement from earlier in the week, authorities said.

Detectives have interviewed several people. At least one driver said he heard shots and believed he saw the light-metallic 2001 Toyota Highlander that Timko was driving slow down near Passyunk Avenue and a pickup abruptly turn off at the exit.

Police said they believed the pickup was a newer, charcoal-colored Ford F150 with a four-door cab and Pennsylvania tags.

The pickup might have come from New Jersey across the Walt Whitman, authorities said, but the expressway in that area can be entered from Philadelphia as well.

It is not clear whether the shooter was in the Ford, police spokesman Lt. Frank Vanore said.

"This is a vehicle of interest," he said. "It may be another witness, but it may be a witness who can unlock this entire thing."

Police ask anyone who knows of a Ford pickup driver who travels that area or who may have been there between 8:30 and 8:45 p.m. Sunday to contact South Detectives at 215-686-3013. Tips will remain confidential, Vanore said.

The shooting most likely occurred at 8:40 p.m., based on the time Timko crossed the Walt Whitman. He was taking his 8-year-old daughter to her mother's home in Norristown, police said.

Before the Passyunk exit, Timko pulled over for an unknown reason, Vanore said. He may have cut off another driver while trying to merge back into traffic.

After some sort of exchange, police believe, Timko turned to shield his daughter as another driver fired at least four shots from a .40-caliber gun and hit him in the forehead. He was able to pull the Highlander to the side of the road before losing consciousness.