FBI agents had Omar Mateen on their radar for years, and long suspected him of jihadist sympathies. But they could never build a case against the man who would go on to conduct the worst mass shooting in US history.
The horrific assault highlights the challenges law enforcement face in preventing "lone wolf" attackers, an urgent concern for authorities now that Isis-inspired jihadists - many radicalised through the internet - are targeting the United States.
Mateen, a 29-year-old American of Afghan descent, died in a gunfight with police after the shooting rampage on Sunday at Orlando's Pulse nightclub that left 49 dead.
He pledged allegiance to Isis (Islamic State) during the attack - albeit in a jumbled 911 message in which he also vowed support for the al-Nusra Front, which is battling Isis in Syria.
The Orlando attack came six months after the mass shooting in San Bernardino, California, where a man and wife opened fire on a Christmas party killing 14 people. The radicalised Muslim couple are believed to have been inspired, if not directed, by Isis.
White House hopeful Hillary Clinton vowed yesterday to set up a team dedicated to detecting and preventing "lone-wolf" attacks.
But experts said such cases raise tough questions about privacy and security, and show just how hard it is to stop radicals who act alone.
The FBI admits it previously investigated the 29-year-old Mateen after he made inflammatory comments to co-workers - but cleared him of extremist ties.
FBI chief James Comey yesterday vowed to "look hard at our own work" to see whether there is something they should have done differently.
"So far, the honest answer is, I don't think so."
Steve Pomerantz, a former assistant director and counterterrorism chief for the FBI, said the agency likely has interviewed "thousands" of people who visit extremist websites or have raised other red flags. But such behaviour is not necessarily illegal.
"As long as I don't cross the line of fundraising for them or recruiting for them, it's not against the law, so what do you do about that? Do you wiretap that guy, and if so for how long? Do you follow him?" Pomerantz said.
"The amount of manpower it would take to do that is beyond the realm of reality. Unless they've actually gone over that line, there are a limited number of things in a free society that you can do about that."
Mateen's case is a potent illustration. The FBI first became aware of him in May 2013 when he was flagged for making comments to colleagues that suggested terror ties.
"We attempted to determine whether he was a possible terrorist, something we do in hundreds of cases all across the country," Comey told a news conference.
The FBI's Miami office opened a preliminary investigation: introducing sources to him, recording conversations with him, following him, reviewing his communication records, and searching government files.
Interviewed twice, Mateen told agents he made the incendiary comments in anger because he thought his co-workers were teasing him as a Muslim. After 10 months, the investigation was closed.
Agents questioned him again the following year over his connection to Moner Mohammad Abusalha, a fellow Floridian who conducted a suicide bombing in Syria.
The two attended the same mosque but again, the probe turned up no ties of any consequence, Comey said. In the light of the challenges of identifying such "lone wolves", observers say responsibility for thwarting them starts with the people who know them.
"For a large majority of offenders in mass-casualty and active-threat situations, others were aware of the grievances that later turned into terrorist plots or active shooting situations," said Michael Masters, senior vice president at the New York-based Soufan Group intelligence consultancy.
Local cops need to improve outreach with local communities and filter reports of suspicious activity up to state and federal authorities, he suggests.
"Unfortunately, I think we are going to see an increasing number of these (attacks), which is why that emphasis on trying to identify individuals and take proactive stances against them is going to be particularly critical at the next stage," he said.
Republican White House hopeful Donald Trump has made a similar point - but alarmed critics by singling out the Muslim community, accusing it of failing to report potential terrorists in its midst.
The Democrat Clinton trained her fire, meanwhile, on the ready availability of guns, which compounds the threat from radicalised individuals: Mateen legally purchased the guns used in the Orlando attack, even though he was once included on a terror screening database.
She reiterated her call for a ban on assault weapons, saying: "It's essential that we stop terrorists from getting the tools they need to carry out the attack."
Killer's mixed allegiances
US authorities yesterday said they had found no direct links between Isis (Islamic State) militants and Omar Mateen, describing him as a homegrown extremist who was inspired by radical Islamist groups.
Mateen had expressed support for multiple armed Islamist movements and people, which "adds a little bit to the confusion about his motives", FBI Director James Comey said. "So far, we see no indication that this was a plot directed from outside the United States and we see no indication that he was part of any kind of network," Comey said in Washington. "We're highly confident this killer was radicalised at least in some part through the internet."
In 911 calls during the rampage, Mateen mentioned support for violent Islamist extremists. He pledged allegiance to Isis and support for the al-Qaeda affiliate al-Nusra Front, which is battling Isis in Syria. Both are Sunni groups.
Comey also revealed that in "inflammatory and contradictory" comments to co-workers in 2013, Mateen had claimed to be a member of Hizbollah, the Shia militia based in Lebanon.
In Syria, Hizbollah supports the Government of Bashar al-Assad, effectively meaning it fights both Isis and al-Nusra Front.
Isis, which controls territory in Iraq and Syria, reiterated yesterday a claim of responsibility, although it offered no signs to indicate co-ordination with the gunman.
What's in a word? The fight over Islamic label
As news of the nightclub shooting in Orlando spread, Donald Trump revived the debate over what to call acts of violence by people inspired or directed by extremist groups such as Isis (Islamic State). Even before it was clear the presumed gunman, Omar Mateen, had expressed an allegiance to Isis, the Republican presumptive nominee declared President Barack Obama should resign if he did not use the words "radical Islamic terrorism" to label the massacre. Obama did not use the phrase. Trump's Democratic rival, Hillary Clinton, didn't either. Here's a look at how Obama, Clinton and Trump have described the Orlando shooting, and others like it, and what the fight over language is really about.
In his statements on the attacks, Obama has used only one of the three words Trump demanded to hear: terrorism. Obama has noted the FBI was investigating the attacks as an "act of terrorism". But it was no surprise that Obama didn't use the "radical Islamic" label. The President has said he is loath to connect such attacks to Islam - even if described as a radical strain. To do so would be unnecessarily provocative, the White House has said, noting the potential to anger Arab allies, alienate Muslims at home and abroad and, in the case of Isis, validate the claims of the enemy. "Isil is not Islamic. No religion condones the killing of innocents," Obama said in September 2014, using another acronym for the group.
Clinton, too, called it an "act of terrorism" and vowed to defeat "international terrorist networks". But yesterday, perhaps in a sign of the political potency in the debate, she said: "Whether you call it radical jihadism, radical Islamism, I think they mean the same thing. I'm happy to say either," Clinton said on CNN. "What I won't do, because I think it is dangerous for our efforts to defeat this threat, is to demonise and demagogue and, you know, declare war on an entire religion. That plays right into Isis' hands." Clinton did not use the word "Islamic", as Trump does, but "Islamism", which specifically refers to Islamic fundamentalism or militancy.
Trump and many Republican allies have cast both Obama and Clinton's parsing as political correctness with dangerous consequences. The failure to prominently connect acts of terror to radical elements within Islam results in a failure to confront the root causes, they argue. "Unless you're going to say it's radical Islamic terrorism and hate ... you're never going to solve it," Trump told CNN yesterday. "And you have Hillary Clinton, refuses to use the words. Now, she doesn't really believe that she shouldn't use it; she's afraid to use it because President Obama doesn't want her to use it." This line tracks with Republicans' broader critique of Obama, and by extension Clinton, when it comes to fighting terrorism. Trump has blasted Obama as "weak" and ineffectual, and suggested he'd be more aggressive in going after Isis.
- AFP, AP