First a 1,200-pound bomb inside a rental van blew a crater into the base of the World Trade Center in 1993, killing six.
Then a rental truck packed with fertilizer exploded in front of the federal building in Oklahoma City in 1995, killing 168 people.
The detonation of a backpack nail bomb a year later inside a public plaza - killing one person in Centennial Olympic Park in downtown Atlanta - was the final straw. Those attacks inspired a new approach for protecting Americans and visiting dignitaries at large events from the growing threat of terrorism and violence on U.S. soil.
The Atlanta Olympics bombing during a joyous, open-air gathering - amid emerging domestic terrorism five years before the 9/11 attacks - would forever change how Americans enjoy major events. It prompted the White House two decades ago to give the U.S. Secret Service the high level of power it has shown in managing security for Pope Francis' visit to Philadelphia next Saturday and Sunday.
Closed highways. Concentric security zones inside a nearly 5-square-mile section of central Philadelphia. Fenced-in viewing areas. Magnetometers and sharpshooters. All are the tools of an antiterrorist apparatus built for prevention.
"You've got to have structure," Secret Service Director Joe Clancy said Friday alongside Jeh Johnson, secretary of the Department of Homeland Security, in remarks in Philadelphia ahead of the pope's arrival. "If you don't . . . you'll have mayhem."
They and other top officials met at a secret communications center established expressly for the global event.
Office buildings will go empty in the days ahead because employees will be blocked from driving into the city's core. Courts and schools will be closed. Hospitals have canceled elective operations. Parts of I-76, the Vine Street Expressway, and the Benjamin Franklin Parkway will also be shut down for more than two days.
"Everything around security and protection of VIPs and major entertainment security is different post-9/11 than it was beforehand. There's no doubt about that," said David L. Cohen, the Comcast Corp. executive who is cochair of the executive leadership cabinet for the weeklong World Meeting of Families, prelude to the papal visit.
"It's a tremendous amount of work," said Pennsylvania Emergency Management Director Richard Flinn Jr.
"I have never seen anything like this," said James Cuorato, the onetime city commerce director who is now president of the Independence Visitor Center on Independence Mall. The Secret Service will take over a large balcony of his building, which faces Independence Hall.
The Pennsylvania State Police will dispatch 1,100 troopers - a quarter of the force - to help.
"We are pulling troopers from all across the state," said Lt. Col. George Bivens, state police deputy commissioner of operations.
The same number were deployed for the Secret Service-led 2009 G20 Pittsburgh Summit with President Obama among nearly two dozen heads of state, Bivens said.
The security effort has a lofty aim: Reduce the risk of an organized or lone-wolf attack as the pope speaks at Independence Hall, says Mass on the Benjamin Franklin Parkway, embraces pilgrims, and parades in the heart of the metropolis over two days.
"That's going to be one of the priorities, the inspired-by, lone wolf who may not necessarily be directed by an overseas power but may be inspired to act out," said retired FBI counterterrorism agent Jeffrey Tomlinson, who supervised the agency's local Joint Terrorism Task Force.
Officials have said little about what, if any, threats have been detected in advance of the pope's visit, even as they have built a major communications infrastructure to monitor threats and be ready for the worst.
On Sept. 15, Philadelphia Police Commissioner Charles H. Ramsey downplayed reports that federal authorities had charged a Camden County teenager in August with plotting a Philadelphia attack against the pope.
The unnamed 15-year-old from Lindenwold reportedly viewed ISIS propaganda over the Internet, made overseas contacts online, and received information through social media on building explosives.
Homeland Security and the FBI circulated a bulletin about the case to law enforcement agencies last month, according to a report by ABC News.
Such interagency coordination was an objective when President Bill Clinton expanded the Secret Service's mission in 1998.
Presidential Decision Directive 62 set out a hierarchy for what would be labeled National Special Security Events.
Once designated an NSSE, an event is secured by the Secret Service, to which all civilian organizers, government agencies, and local, state, and federal law enforcement troops must report. The FBI takes charge of intelligence and investigation; the Federal Emergency Management Agency leads any mass-casualty response.
When NSSE was created, the focus was to prevent a repeat of Atlanta at the Salt Lake City Winter Games in 2002.
"Many of the problems in Atlanta reflected how slow we were as a nation to begin to recognize that terrorism was becoming a security issue inside the United States," Mitt Romney said in 2004 testimony to the Senate Commerce Committee. Romney led the Salt Lake City Games organizing committee.
In Atlanta, more than 50 federal, state, and local agencies were involved in securing the Olympics. But there was no clear command structure for preventing an attack, only for responding to one.
Philadelphia is no stranger to NSSEs, although the pope's visit is the largest event security operation ever undertaken by the Secret Service, involving about 70 agencies.
Restrictions surrounding the visit, to many in the city, have been viewed as excessive.
The pope safely celebrated Mass earlier this year before millions in the Philippines without extreme security measures in place. Yet vast areas of Philadelphia are being shut down, leaving many wondering why.
Tomlinson, the retired FBI counterterror agent, said the U.S. civil freedoms that require court orders for telephone wiretaps, for example, pose a challenge that does not exist in more authoritarian states. The lack of a national police force, and the nation's decentralized system of law enforcement agencies on local, state, and federal levels, is another reason for the need to coordinate this way, he said.
The government said it did not yet know how much all of the security associated with this NSSE would cost. Only $4.5 million a year is budgeted annually in Washington for NSSEs. No law requires that federal money be sent to help local and state agencies shoulder the costs.
David Beach, local Secret Service agent in charge, said the roughly 100 square city blocks being secured here would make Philadelphia's the largest NSSE area ever.
"This will be our 51st," said Beach, who has worked on about 20 NSSE events since 2004. "This is the most challenging one" of his career. And yet, with all of the planning, no one can predict how quickly the security team will respond to a threat.
"I think the biggest challenge is going to be the last-minute communication of a threat, and how you respond to that threat," Tomlinson said. "Do we capture it, analyze it, and act on it quickly enough?"