Federal and local authorities are continuing to investigate a tipster's claim that Edward Archer, the man charged in the shooting of Philadelphia Police Officer Jesse Hartnett, was part of a small group of men with radical beliefs, officials said Monday.

Homicide Capt. James Clark said that after a weekend of investigating, authorities have been unable to substantiate the tip, delivered to police by a woman who warned that "the threat to police is not over," and said Archer was associated with a group of three other men more radical than he.

"We're investigating it, but we haven't found anything yet," Clark said.

He and other officials said they take such information seriously and are actively investigating Archer and his background.

"You have to, with these types of situations," said Lt. John Stanford, a department spokesman. "You don't want to take anything for granted or overlook anything."

Archer, 30, of Yeadon, ambushed the officer late Thursday, emptying the magazine of a stolen 9mm police firearm into Hartnett's patrol car and hitting him three times in an arm. Archer later confessed to carrying out the shooting "in the name of Islam," and told investigators he had pledged allegiance to the Islamic State.

The department's Homeland Security Unit and the FBI's Terrorism Task Force are working on the investigation.

The tip about Archer's associations was fielded Saturday night, when a woman approached a police officer on the street, police said.

In response, the department ordered all officers to patrol in pairs until further notice.

"It's just as a safety precaution," Stanford said. "That's not the norm for every single day, for officers to be partnered up."

He said the department had been shaken by Hartnett's shooting. Still, he said, "at the end of the day, we have a job to do, and in the back of every officer's mind, they know that the danger exists. Our cops are still lacing up their boots and going out and serving the citizens of this city."

Archer was charged over the weekend with the attempted murder of Hartnett, 33, an 18th District officer, who was patrolling at 60th and Spruce Streets when Archer opened fire.

Hartnett was seriously wounded and is still hospitalized.

Investigators spent the weekend digging into Archer's background, seizing evidence from his mother's home in Yeadon and combing through his online history.

"We'll work on all the leads and tips and see if it becomes anything," Clark said of the joint local and federal investigation.

Archer's mother said last week that her son had suffered several concussions from playing high school football and from a moped accident. Lately, she said, he had been "hearing voices in his head." He needed medical help, she said.

Valerie Holliday described her son as a devout Muslim and said she was shocked at what police say he did. She said she had no idea where he might have gotten a gun.

Reached again on Monday, she declined comment.

Those who knew Archer in his high school days said they were shocked by the charges against him.

John Leary, a former football coach and principal at Penn Wood High School, remembered Archer as a "good kid" who was a C student in the classroom and a determined athlete on the playing field.

"He wasn't the most talented kid," said Leary. "He stuck it out. He was respectful to the coaches and other players. He had fun."

Archer played quarterback and defensive back, Leary said, but he could not recall if Archer suffered any concussions while on the team.

Raymond Bland, a fellow teammate at Penn Wood who has known Archer since the ninth grade, recalled him as a mild-mannered loner with a passion for football.

"He was a freelancer who did his own thing," said Bland. "He was not upset about it, he was chill."

He said Archer liked to watch Dragon Ball Z, a Japanese anime television series. He stayed clear of fights in high school and was respectful of the teachers, Bland said. He did not recall Archer as particularly religious.

The two lost touch after high school. Bland went to college, then joined the Marines and was deployed to Afghanistan. He is now studying communications at Penn State Brandywine.

About a year ago, Bland said, he ran into Archer at a Lansdowne basketball court and was shocked to see him dressed in Muslim garb.

"That was when the whole religious thing came up," he said. Archer said he had been doing a lot of travel, including a trip to Egypt. He said he was getting deeper into his faith and that he had married, Bland said.

Looking back on the conversation, Bland said, "Nothing would lead me to believe he would be capable of such a heinous crime."