In a bulletin sent to law enforcement agencies obtained by ABC News, the agencies warn that they should be on alert due to increased al-Qaeda propaganda, though there are currently no specific threats.
"Would be prudent for the intelligence community and the law enforcement community to be on the lookout for anything other than normal because it's not a normal day," said Jack Tomarchio, a former Homeland Security official. "It's the 20th anniversary of one of the greatest disasters in our domestic history."
RELATED: Philadelphia-area 9/11 Memorials: See map of more than 100 locations
Some area agencies are preparing for an elevated threat environment. Officials point to recent events that could potentially spark a terrorist act like American troops pulling out of Afghanistan, the return of the Taliban in that country, and the suicide bombing at the Kabul airport.
"It's a warning to local law enforcement that some fringe group or possibly a domestic terrorist or even a lone wolf may decide that this is a good time for him to make a statement or them to make a statement in the anniversary of the 9-11 attacks," added Tomarchio.
A statement from the Philadelphia police says they are monitoring the situation. And while there are no known specific threats to Philadelphia, the department is prepared to activate with additional personnel throughout the city in the coming days.
"The agencies are doing the right thing, the safe thing, they're doing the professional thing," Tomarchio said.
Americans are being urged to live their lives on this solemn anniversary and remember those who were lost.