A message flashed across Philadelphia Police Chief Inspector Joseph Sullivan's phone as he sat in a Center City conference room Thursday: A suspicious package had been spotted blocks away.

As it turned out, the bag was harmless, the tip familiar.

In the month after the Boston terror attack, Philadelphia police responded to more than 300 such reports, with 32 just on the day of the Broad Street Run, Sullivan said.

Bomb-squad officers even built and detonated a backpack bomb identical to the one that killed three people in Boston.

"Because, if we know how to build it," Sullivan said, "we know how to take it apart."

The disclosure came during a counterterrorism round table hosted by the World Affairs Council on Thursday.

With the Boston bombing as a backdrop, Sullivan joined Philadelphia FBI Special Agent in Charge Edward J. Hanko and Pennsylvania Homeland Security Director Thomas Minton III to discuss what agencies are doing, or should be doing, to thwart such attacks.

Asked whether Philadelphia had ever been specifically targeted, Hanko said he believed that every major U.S. city had emerged in terror chatter at some point.

Given the city's importance in American history, he said, it's almost a given that "our enemies are thinking about doing something here."

Boston police have complained that they were not told about a tip to the FBI that one of the bombers, Tamerlan Tsarnaev, may have been radicalized after traveling to his Chechen homeland.

Sullivan, whose department has officers on the FBI's Joint Terrorism Task Force, said there was "no issue regarding the sharing of information" here.

Hanko said the public rarely hears about plots that are foiled. "The terrorists only have to be successful once," he said. "We could be successful 1,000 times - and we have been."

Federal money has helped fund a state intelligence "fusion" center in Harrisburg that Minton said had been highly successful at sifting through local reports and passing relevant information to the agencies that need it.

He said his office had also organized training exercises and coordinated response plans with agencies throughout the state, including one tied to the forthcoming U.S. Open.

But shrinking resources - particularly federal money - could affect the ability to respond.

Hanko noted that there would never be enough agents or officers to protect the country, and if there were, it wouldn't look like America. Instead, stopping the next terror plot increasingly depends on a watchful and engaged public, the men said.

Said Sullivan: "It's going to be people in the community who prevent the next attack."

Contact John P. Martin at 215-925-2649, jmartin@phillynews.com or @JPMartinInky on Twitter.