Rezwan Ferdaus, 26 and living in Ashland, had acquired explosives from undercover federal agents to further his plan to blow up the Pentagon before he was arrested in September 2011.
Alexander Ciccolo, 24, had allegedly acquired four guns from a witness cooperating with the FBI and built several Molotov cocktails before agents were able to arrest him last July in Adams, where he lived.
And authorities were able to arrest David Wright, 26, of Boston, and Nicholas Rovinski, 25, of Rhode Island, only after investigators obtained evidence that a coconspirator wanted to kill police officers.
In each of these local cases, law enforcement officials met a legal standard for probable cause that eluded Florida investigators for years in the case of Omar Mateen, before he launched his deadly shooting spree at an Orlando nightclub. Meeting that standard required authorities to spend long hours watching suspects and demonstrating the likelihood that danger loomed.
Analysts say the Orlando case demonstrates the difficulty federal investigators face in identifying a true threat. When does a person who is deemed suspicious cross the threshold and demonstrate probable cause that he or she will carry out an attack?
Mateen, whose assault on the Pulse nightclub in Orlando killed 49 people, "was flirting with this ideology for years," Seamus Hughes, the deputy director of the Program on Extremism at George Washington University, said in a recent interview. With terrorist groups such as the Islamic State continually urging people to carry out attacks, he said, the pressure on the FBI to monitor such cases is increasing.
"It's forcing FBI agents to make tough judgment calls and quicker than they have in the past," Hughes said. "There's always going to be another case that comes on the pile."
Security analysts, such as Hughes, say it is too early in the investigation to say whether the Mateen case was handled appropriately but point out that questions have been raised: How was Mateen able to purchase guns after being on a watch list without raising further suspicion? How much information about Mateen did the FBI share with local authorities?
Mateen had twice been placed on a terrorism watch list, in 2013 after co-workers reported that he was claiming connections to Al Qaeda, and again in 2014 after authorities learned he was associated with an American man from a Florida mosque who went to Syria and blew himself up for the Nusra Front, Al Qaeda's branch in that country. Several Syrian troops were killed in the suicide bombing, believed to be the first by an American in Syria.
Investigators tracked Mateen's foreign travels and interviewed him twice, yet were unable to detect any evidence that he would carry out an attack.
Authorities often set up sting operations that use an informant, a cooperating witness, or an undercover agent to pretend to conspire with a suspect in an attempt to learn his or her true intentions — as they did with Ciccolo and Ferdaus. They say they attempted the same strategy with Mateen, but that failed to yield evidence that he was planning any violence.
Then nearly two years later, with no warning, he carried out what has been called the worst mass shooting in modern US history.
FBI Director James B. Comey told reporters that the FBI will investigate its handling of Mateen but said a preliminary review shows that the agency could not have done anything to stop the attack. Mateen was placed on a watch list, but the Department of Justice and civil liberties standards require FBI officials to remove names from the list if they find no probable cause that the people plan to commit a crime. Mateen was taken off the list.
"We are looking for needles in a nationwide haystack, but we are also called up to figure out which pieces of hay might someday become needles," he said.
Analysts say the FBI faces a tough challenge in determining whether to confront people who have not committed a crime but have embraced the ideology of terrorist groups.
Hughes said another important question is whether groups like the Islamic State that encourage followers to directly attack virtually any targets in their home country have been so successful that they have reduced the time it would take from "going from mobilizing to action."
"It doesn't take as much advanced planning" as past attacks, Hughes said.
Max Abrahms , a Northeastern University professor and counterterrorism analyst, said investigators face more obstacles identifying true terrorists in the United States because they tend to work alone, rather than in the networks seen in Europe. He said US authorities have been most successful when deploying sting operations.
He and other analysts cautioned, however, against targeting specific communities or enticing people who would not have participated in a plot if they had not been provoked.
"Certainly we have concerns . . . that some of the use of informants have been targeting not people who are actual threats, but rather people who are susceptible or vulnerable to being persuaded to engage in a plot that the FBI conceived, and gave materials and encouragement for," said Shannon Erwin of the Muslim Justice League, a Boston-based civil rights group.
Joel Day, an international counterterrorism professor at the University of Massachusetts Lowell, said what is clear is that groups such as the Islamic State, also known as ISIS, present a new front in the war on terror. People who might not have acted on their beliefs before have latched on to the Islamic State-inspired message to attack, he said, whether it be at a political establishment, a gay nightclub, or a workplace.
"ISIS has transformed into a brand of global ultraviolence, and it allows people to kill without regard to political considerations, religious considerations," he said.
The local cases that resulted in arrests before violence reflect the different ways in which a threat can be identified.
In the case of Ferdaus, the Ashland man had devised a plan to send explosive-laden remote-control airplanes into the Pentagon, and to shoot everyone who tried to flee the building. Ferdaus was sentenced in 2012 to 17 years in prison.
Ciccolo allegedly planned to shoot people at an unnamed local university and discussed the plan with an informant, who provided the weapons. He is awaiting trial.
Rovinski and Wright were under investigation for months on suspicion they were plotting to kill anti-Islam activist Pamela Geller. They were arrested, however, only after Usaamah Abdullah Rahim — Wright's uncle — was shot by police officers last June when he allegedly attacked one of them with a knife. Rovinski and Wright were later indicted and are awaiting trial on charges that they tried to establish a local terror cell.
In the well-documented case of the 2013 Boston Marathon bombings, investigators questioned Tamerlan Tsarnaev in 2011 after Russian authorities warned that he was embracing extremist Islamic views, though they never followed through with an investigation. Two years later, Tsarnaev and his younger brother, Dzhokhar Tsarnaev, killed three people and injured more than 260 at the Marathon finish line.
Harold Shaw, special agent in charge of the FBI's Boston office, would not discuss any local investigation or comment on how the agency monitors suspects, saying only, "We will be as aggressive as we can in utilizing every tool available to us to gather the intelligence to understand a threat and disrupt it."
He added, however, that any law enforcement action needs to be based on an "overt act" and said authorities cannot arrest based solely on an ideology.
"It really comes down to what constitutes an overt act," he said.