Investigators have been working to determine whether Archer, 30, of Yeadon, had contact with or help from anyone with ties to terrorist groups.
No conclusive connections had been found in the week since the investigation began, law enforcement sources said. Still, federal agents continue to review Archer's online activity, recent communications, and trips he took to Saudi Arabia in 2011 and Egypt in 2012.
The FBI is "investigating the possibility of it being a terrorist attack, not that it definitely is," a high-ranking police official said late Wednesday.
Authorities also said Wednesday that they believe they have identified the three men mentioned in an anonymous tip to officers Saturday that warned "the threat to police is not over."
According to a police report, a woman approached an officer on the street and said Archer regularly associated with the men, whose religious beliefs she described as radical.
Agents continue to look into the backgrounds of the men, said a federal law enforcement source.
Comey's statements Wednesday during a routine visit to an FBI field office in Pittsburgh came amid growing debate on how to describe Archer's actions.
"We are investigating that as a terrorist attack," he told reporters. He said ISIS has recently focused on "crowdsourcing" terrorism and trying to motivate "troubled souls" to commit violence on their own without direct contact with the group.
Mayor Kenney has been criticized by some - including Sen. Marco Rubio of Florida, a Republican presidential candidate - for saying in the hours after Hartnett was shot that the crime was "not an act of religion." He has stood by those statements.
"That act of that terrible man in almost assassinating our police officer was an individual act," the mayor said Saturday.
Archer has been charged with attempted murder in the shooting of Hartnett, who was ambushed while patrolling at 60th and Spruce Streets in West Philadelphia.
In a video that drew national attention, Archer, dressed in a long white tunic, could be seen approaching the driver's side door of the officer's patrol car. From point-blank range, he fired more than a dozen bullets into the vehicle from a 9mm Glock that had been stolen from the home of another Philadelphia officer.
Hartnett, with three gunshot wounds to his arm, managed to get out of his car and return fire, wounding his assailant.
Archer's mother, Valerie Holliday, has attributed her son's violent act to erratic behavior stemming from head injuries she said he suffered playing football. She has described him as a devout Muslim.
Hartnett, 33, continues to recover at Penn Presbyterian Medical Center.
The continued threat against Philadelphia police has drawn national attention and brought condemnation from members of Congress. Sen. Pat Toomey (R., Pa.) will meet with FBI officials in Philadelphia on Thursday to discuss their ongoing investigation. Sen. Robert P. Casey Jr. (D., Pa.) is scheduled to meet with Police Commissioner Richard Ross on Thursday for a briefing on the investigation of Archer and other potential terrorist concerns.
Staff writers Robert Moran and Aubrey Whelan contributed to this article.