WASHINGTON (AP) — The man accused in the Manhattan bombing was flagged for an interview with Customs officials after returning from an overseas trip, accused two years ago of stabbing his brother, and had once been angrily described by his father as a terrorist.

But none of that was enough to keep Ahmad Khan Rahami on the FBI’s radar for long — and it’s not clear it should have been.

The case illustrates the difficulties investigators face in identifying and stopping individuals who might be violent extremists. Constitutional protections permit people to hold abhorrent views — and even view jihadist videos — without automatically coming under investigation. Finite resources mean that more intrusive investigative tactics, such as round-the-clock surveillance, are generally reserved for those thought to be conceiving an attack. And preliminary FBI inquiries like the one conducted in Rahami’s case are, by design, not meant to be open-ended.

Some questions and answers about the investigation into Rahami and how the FBI conducts terrorism investigations.

Q. HOW DID RAHAMI FIRST BECOME KNOWN TO THE FBI?

A: The FBI says it opened an assessment on Rahami in 2014 following a domestic disturbance in which he was accused of stabbing a brother, when the agency was alerted to his father’s concerns that he might be a terrorist.

The elder Rahami told The Associated Press that he told agents about his son’s apparent radicalization, but the FBI says he never shared those concerns at the time, and law enforcement officials say he backed off his earlier characterization of his son.

The FBI says it checked its databases and reviewed Rahami’s travel records and found nothing tying him to terrorism. The review was closed with no further action.

Rahami, an Afghan-born naturalized U.S. citizen, traveled in recent years to Afghanistan and Pakistan. The length of one trip to the region automatically triggered a secondary interview with Customs and Border Protection following his return in 2014. Details of his trip and his interview were forwarded to the agency intelligence center charged with identifying potentially high-risk travelers, according to a U.S. government official who was not authorized to publicly discuss details of the ongoing investigation and spoke on condition of anonymity. The advisory was not sent as an urgent alert but a more routine notification that was then distributed to law enforcement and intelligence agencies.

While an extended stay or repeated trips to a region such as Pakistan or Afghanistan may raise questions among law enforcement, including CBP, such travel is not illegal and doesn’t automatically arouse suspicion.

It’s not yet clear that any obvious red flags were missed, through the FBI is certainly looking at its past interactions.

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Q: WHAT STEPS ARE THE FBI PERMITTED TO TAKE IN TERRORISM INVESTIGATIONS?

A: That depends entirely on how much information investigators have to indicate terrorist activities.

An assessment, the type of inquiry used in Rahami’s case two years ago, is the most basic and least-intrusive of FBI reviews and is meant to last only a matter of weeks. Agents checking out a tip may peruse publicly available records, check government documents and request information from the public. Though agents may seek extensions, assessments are meant to resolve in a matter of weeks with agents either closing out the review or finding sufficient grounds to continue the review.

“Assessments are supposed to be quick-hit investigations to determine if there was anything that would lead you to probable cause to investigate a person,” said David Gomez, a retired FBI official who oversaw counterterrorism investigations in the Seattle field office.

The system, laid out in the Domestic Investigations and Operations Guide, is meant as a triage to help discern between dead-end tips and more compelling ones. If the FBI opened a full investigation each time it conducted an assessment, Gomez said, there’d be long-term inquiries on many people when there’s minimal basis for suspicion — a prospect Gomez said would be “repugnant to people.”

More formal investigative steps — such as surveillance and recording of phone calls — require a higher level of approval and a firmer basis for suspecting wrongdoing.

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Q: HOW IS IT THAT INDIVIDUALS WHO ARE KNOWN TO THE FBI LATER GO ON TO COMMIT VIOLENCE?

A: This is certainly not the first instance in which someone who was at one point on the FBI’s radar went on to be accused of an attack.

The FBI investigated Omar Mateen, the Orlando nightclub shooter, for 10 months in 2013 after boasting of mutual acquaintances with the Boston Marathon bombers and making statements to co-workers that suggested he had radical, violent intentions. Agents conducted surveillance, contacted confidential informants and even undertook multiple interviews but ultimately found nothing to justify continued scrutiny. Agents looked into him again the next year as part of a separate investigation into a suicide bomber who attended the same Florida mosque.

The FBI was also aware of Elton Simpson, one of the two gunmen who attempted an attack on a Prophet Muhammad cartoon contest in Garland, Texas last year. Simpson had previously been prosecuted in Arizona in a terrorism-related case and was placed on probation for lying to a federal agent. He came back onto the government’s radar soon before the May 2015 violence because of postings on social media.

Law enforcement officials say it’s unrealistic to expect indefinite investigations of everyone who was once on the FBI’s radar, particularly as agents move on. FBI Director James Comey said agents have “north of 1,000” people who are under scrutiny and somewhere on the spectrum of radicalization.

“We are looking for needles in a nationwide hay stack, but we are also called upon to figure out which pieces of hay might some day become needles,” Comey said in June when discussing the agency’s past interactions with Mateen. “That,” he said, “is hard work.”

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Associated Press writers Eileen Sullivan and Alicia A. Caldwell contributed to this report.