Police said the gunman, 30, confessed to the attack "in the name of Islam." The weapon he used had been stolen from another police officer's home.
The Philadelphia Police Counterterrorism Unit and the FBI were investigating allegations from the tipster that the man who shot Hartnett was associated with a small group of men with radical beliefs, and warned that "the threat to police is not over."
Specifically, she said there were three other men, more "radical" in their beliefs than Archer.
Police spokesman Lt. John Stanford said the department and federal agencies take such information seriously.
Police said the tipster stopped the officer on the street to relay information. Archer, 30, of Yeadon, was arraigned Saturday and ordered held without bail, his preliminary hearing set for Jan. 25. Court documents show he is being represented by a public defender.
Archer was captured on video approaching Hartnett's patrol car and firing at him without warning, at one point leaning into the squad car's open window to shoot at the officer.
Hartnett, 33, was in critical but stable condition, recovering Sunday at Penn Presbyterian Medical Center.
In his confession, Archer pledged his allegiance to Islamic State. Police Commissioner Richard Ross said Archer told investigators he believed police enforced laws counter to the Quran.
Investigators combed through Archer's residences in Philadelphia and Yeadon, trying to piece together his online activity for any possible connection with terrorist groups.
Ross said investigators had informed him Saturday that they so far had found no such connection.
Of interest were two trips Archer made to the Middle East, in 2011 and 2012: one to Saudi Arabia for the Hajj, a pilgrimage to Mecca, and a 10-month visit to Egypt for reasons unclear to authorities.
The tipster told police that Archer was associated with three other men and that Archer "is not the most radical of the four," police sources said.
Stanford said the tipster "alleged the defendant had an affiliation to a group with radical beliefs." Police did not immediately specify the nature of those alleged beliefs.
Police sources said the tipster said the three men in the group "still frequent the 6100 block of Pine," near where Hartnett was shot. She warned police to "be careful."
The woman said Archer had attended the Masjid Mujahideen mosque on 60th Street near Pine. She contended that an imam there had "lied" when he said Archer was not associated with the mosque.
But Asim Abdur Rashid, the imam of Masjid Mujahideen on South 60th Street, said Sunday night that he did not initially recognize the man by the name of Archer.
"He was a frequent member of the masjid. But I did not know him as Archer, and I did not recognize his picture.
"I know him as Abdul Shaheed. . . . We don't call each other by the names we had before we converted to Islam," Rashid said.
The female tipster told police that Archer also attended a mosque at 45th and Walnut Streets, where he "became more radical."
The closest mosque to that address is Masjid Al Jamia on the 4400 block of Walnut Street. A man and a woman who were praying inside Sunday afternoon said Masjid Al Jamia does not teach or condone radical Islamic activity. Neither would give a name.
They said their mosque is the North American headquarters for the Association of Islamic Charitable Projects.
"The AICP is not a terrorist organization at all and the teachings here refute that ideology," the man said. "ISIS kills our people."
The woman suggested that Archer could be mentally ill - something the accused man's mother said in an interview with The Inquirer after Hartnett was shot.
"There's obviously a chance he could have been touched," the woman at the mosque said. "These are not the teachings of the prophet."
On the website of the mosque at 4431 Walnut St., the AICP expressed "great sorrow" at "news of the heinous ambush of officer Jesse Hartnett . . ."
"We all stand united in dealing with the pain of this calamity as well as in our strong and unwavering stance in denouncing and combating terrorism."