Philadelphia police probe possible extremist ties pledged allegiance to Islamic State.

Philadelphia police said on Sunday they were seriously pursuing a tip that the man suspected of shooting and wounding a police officer had ties to a group with "radical beliefs."

Police are working with the Federal Bureau of Investigation to verify the credibility of the tip related to the ambush shooting on Thursday, which led to charges of attempted murder, assault and aggravated assault against Edward Archer.

He later confessed to the attack and said he pledged allegiance to Islamic State, police said.

Police said in a statement on Facebook on Sunday that a policeman was stopped on the street on Saturday night by a citizen who began talking about Archer and the shooting that seriously wounded officer Jesse Hartnett.

"The citizen alleged the defendant had an affiliation to a group with radical beliefs," police said. "The Philadelphia Police Department has alerted all department personnel of this matter and will continue to have officers work with a partner until further notice."

Archer, 30, is accused of approaching Hartnett, 33, and firing 11 rounds, some at point-blank range, through the police car window. Three shots struck the officer in his arm.

Video of the attack on Hartnett shows a man in a long robe similar to the traditional dishdasha, or thobe, often worn by Muslim men in parts of the Middle East, as he opened fire on the police car. Police officers nearby arrested him.

The Philadelphia Inquirer and television station NBC10 in Philadelphia quoted police sources on Sunday as saying that according to the tip-off Archer was associated with three other men and was not the most extreme of the group. The sources said police were warned the threat to them was not over, the two media outlets said.

Police did not immediately return a call from Reuters for comment.

Anxiety over the threat posed by Islamic State in the United States has risen in recent months. A Muslim couple inspired by the militant group killed 14 people on Dec. 2 in San Bernardino, California, just weeks after gunmen linked to the group killed 130 people in Paris.

People who knew the gunman told Reuters he was a devout, quiet Muslim who became more "combative" after trips to Egypt and Saudi Arabia.

They said he began expressing interest in Islam as a teenager, played in a local Muslim football league and worked on construction jobs.

The imam of a local mosque where Archer worshipped, Asim Abdur Rashid, said Archer’s Muslim name was Abdul Shaheed. He said Archer had attended the West Philadelphia mosque, known as Masjid Mujahideen, for at least five years. He saw him as recently as last week.

“He was intelligent, and he was a regular dude,” Rashid said.

Rashid added that Archer regularly attended morning and evening prayers, in addition to midday Friday prayers.

FBI Special Agent Eric Ruona said Archer had traveled to Saudi Arabia in 2011 and Egypt in 2012 - a trip that a family friend said had changed him.

"He became more drastic. More combative," said Jannah Abdulsalaam, who asked only to be identified by her Muslim name. "He was kind but I noticed that change."