Orlando – Even as FBI investigators peered deeper Tuesday into the life of the Orlando nightclub gunman, the agency faced its own internal reckoning over whether warnings signs were missed during a 10-month probe of the shooter three years ago.

The expected review of the FBI files into Omar Mateen – including why the investigation was effectively closed – adds another layer of questions into the deadliest mass shooting in U.S. history.

However, it also underscored the challenges confronting authorities trying to isolate individual potential threats amid wider concerns over the reach of terror groups such as ISIS – which allegedly inspired a stabbing attack in Paris late Monday that killed a couple working for police agencies.

In this matter, the Federal Bureau of Investigation will “look at our own work, to see if there is something we should have done differently,” the agency’s Director, James Comey, said Monday after details emerged of the past FBI review of Mateen, who left 49 people dead in a gay club Sunday before being killed when police stormed the site.

“So far, I think the honest answer is: I don’t think so,” Comey added. “We will continue to look forward in this investigation and backward.”

The FBI investigated Mateen beginning in 2013, putting him under surveillance, recording his calls and using confidential informants to gauge whether he had been radicalized after the suspect talked at work about his connections with al-Qaeda and dying as a martyr.

On another hand, Mateen’s apparent previous connections to the Pulse nightclub and gay online sites brought new signals into the investigation. At least two people told The Washington Post that Mateen had visited the popular club, and one of the witnesses – Kevin West, a 37-year-old Navy veteran – said Mateen had made contact with him on “Jack’d,” which is a dating app for men.

The political fallout from Orlando, meanwhile, moved onto the world stage even as it overshadowed the U.S. presidential race.

The top U.N. human rights official, Zeid Ra’ad Al Hussein, called on American authorities to adopt “robust gun control measures.”

“It is hard to find a rational justification that explains the ease with which people can buy firearms, including assault rifles, in spite of prior criminal backgrounds, drug use, histories of domestic violence and mental illness, or direct contact with extremists – both domestic and foreign,” the chief of the U.N. High Commissioner for Human Rights said in a statement.

As part of the FBI investigation, Mateen was placed on a terrorism watch list and interviewed twice before the probe was closed in March 2014 because agents concluded he was not a threat, FBI director Comey said Monday in an interview with reporters at bureau headquarters.

Several months later, in July 2014, Mateen surfaced in another investigation into the first American to die as a suicide bomber in Syria, a fellow Floridian. Again, investigators moved on.

It was the third time – following the Boston Marathon bombing in 2013 and a planned attack last year on a contest to draw the prophet Muhammad – that someone who had been scrutinized by the FBI later carried out a terrorist attack.

Comey said that during the three-hour standoff the gunman had with Orlando police officers, there were three different 911-related calls with him. The gunman called 911 at about 2:30 a.m., about half an hour after opening fire, and then hung up the phone.

Mateen then called a second time and spoke briefly to a dispatcher before hanging up again, and then the dispatcher called him back and they spoke briefly.

“During the calls, he said he was doing this for the leader of ISIS, whom he named and pledged loyalty to,” said Comey.

However, Comey said there were no signs that Mateen was directly tied to any kind of network, and he added that it remained unclear exactly which extremist group he supported.

In addition to pledging allegiance to the leader of ISIS, Comey noted, Mateen referred to a link to its rival, al-Qaeda – an American’s suicide bombing in Syria.

He also expressed solidarity with the Boston Marathon bombers.

Law enforcement officials in Florida, meanwhile, offered a new accounting of the shootout.

Orlando Police Chief John Mina said that police first encountered Mateen shortly after the initial gunfire at about 2 a.m., when an off-duty officer working at the club exchanged shots with Mateen.

Additional officers called to the scene soon joined in another gun battle, at which point Mateen retreated farther into the building and, eventually, into a bathroom.

In a news conference Monday, Mina said storming the building “was the right decision to make” because police thought other lives might be in danger.

Authorities said the investigation into Mateen has expanded to look at other people and stretches from Florida to Kabul.

Mateen’s family is originally from Afghanistan, but he was born in New York and lived for many years in Florida.

Comey said Mateen, who worked as a contract security guard at a local courthouse, claimed in 2013 to co-workers that he had family connections to al-Qaeda and was a member of the so-called Hezbollah, two opposing terrorist groups that have clashed repeatedly in Syria.

The FBI director called the comments “inflammatory and contradictory.”