Gunman Edward Archer of Philadelphia approached Officer Jesse Hartnett, 33, shortly before midnight and fired 11 rounds, three of which hit the officer in his arm. Police released still images from surveillance video that showed gunman dressed in a long white robe walking toward the car and firing, eventually getting close enough to shoot directly through the window.
Hartnett chased Archer, who was arrested by responding officers and later confessed to the attack, saying he had carried it out "in the name of Islam," police officials told reporters.
"He has confessed to committing this cowardly act in the name of Islam," Ross told a press conference, adding that the man also referenced Islamic State militants.
Philadelphia Police Captain James Clark added, "He said he pledges his allegiance to Islamic State, he follows Allah and that was the reason he was called on to do this."
U.S. officials have been on high security alert following a series of Islamic State-linked attacks at home and abroad over the last few months.
In November, gunman and suicide bombers affiliated with Islamic State killed 130 people in a series of coordinated attacks in Paris. Last month a married couple fatally shot 14 people in San Bernardino, California, in an attack inspired by Islamic State militants.
Those concerns have led to calls by some Republican governors and presidential hopefuls to restrict the admission of Syrian refugees fleeing that country's long civil war.
Philadelphia Mayor Jim Kenney, a Democrat sworn in on Monday, told reporters he did not believe Archer's actions reflected Islamic thinking.
"In no way shape or form does anyone in this room believe that what was done represents Islam," Kenney said. "This was done by a criminal with a stolen gun."
NO SIGN OF CONSPIRACY
There was no evidence as yet that the shooter had worked with anyone else, Ross said.
"He was savvy enough to stop just short of implicating himself in a conspiracy," Ross said. "He doesn't appear to be a stupid individual, just an extremely violent one."
Officer Jesse Hartnett, 33, who pursued the shooter, was taken to Penn Presbyterian Hospital and will require several surgeries.
"We're just lucky, that's all I can say," Ross told reporters. "I can't even believe that he was able to survive this."
The shooter used a gun that had been stolen from a Philadelphia police officer's home several years ago, but not by the shooter, Ross said.
"We know it was stolen, how many hands it may have passed through in the last couple of years, we do not know," Ross said.
In New York City, where two police officers were shot dead in their patrol car in a December 2014 attack by a man angry over police killings of unarmed black men, the police department issued a memorandum urging officers to "exercise heightened vigilance and implement proactive measures" in light of the Philadelphia shooting.