Police issued a statement Sunday saying a citizen had stopped a Philadelphia police officer on the street and claimed the suspect, Edward Archer, had "an affiliation to a group with radical beliefs."
The statement doesn't name the group or say where it is located or whether the alleged association is past or present.
The tipster told police the group was made up of four people, including Archer, who was the least radical of its members, said John McNesby, the president of the Philadelphia Fraternal Order of Police, who viewed the police report on the information the tipster provided. That person also warned that the threat isn't over, according to McNesby.
The department and the FBI are looking into the claim "in an effort to verify the credibility of the information given," police said.
Archer allegedly told police after he ambushed Officer Jesse Hartnett that he had pledged allegiance to the ISIS terror group, according to Capt. James Clark, commander of the Philadelphia Police Department's homicide unit.
Hartnett was shot three times in the arm Thursday while sitting in his patrol car.
Images released by police from surveillance video show the gunman -- wearing an ankle-length, white garment -- on a crosswalk taking aim and firing at the patrol car at close range.
In one photo, the shooter is standing next to the car with his arm through the lowered driver's side window, close enough to be shooting at point-blank range.
Despite bleeding heavily, Hartnett was able to get out of his car and shoot Archer, whom other officers then arrested, authorities said.
Hartnett survived the attack with what police Commissioner Richard Ross Jr. described as "very serious injuries."
McNesby said Monday that Hartnett, who was scheduled to undergo another surgery, is "continuing to progress."
"He's going to have a long road ahead of him," he said.
Police said Archer was armed with a Glock 17 pistol stolen from a police officer's home in 2014.
According to Clark, Archer told investigators: "I follow Allah. I pledge my allegiance to the Islamic State and that's why I did what I did."
The shooting in Philadelphia is the fourth attack in the United States believed to have been inspired by ISIS, including the December shootings
that left 14 dead in San Bernardino, California, the shootings outside a cartoon contest
in a Dallas suburb in May in which images of the Islamic Prophet Mohammed were being displayed, and a hatchet assault
on four police officers in New York in October 2014.
Officials have said they do not know whether the latest attack was part of a broader conspiracy or whether Archer had contact with terrorists.
Philadelphia police said all officers will continue to work in pairs until further notice.