The FBI let the Orlando mass-shooting suspect slip through its grasp despite interviewing him twice since 2013 due to a lack of evidence to hold him, a troubling fact that will pressure officials struggling to detect lone terrorists without eroding basic civil liberties.

The FBI is investigating Sunday’s killing spree at a gay dance club in Orlando, Florida, as an act of terrorism after 29- year-old shooting suspect Omar Mateen killed 49 people using an assault weapon and a handgun. Mateen, who called 9-1-1 as he began the assault to claim allegiance to Islamic State, was killed in a shootout with police.

While the U.S. has made progress in countering groups like Islamic State on the ground overseas, technology allows their radical ideology to reach across borders and lure true believers, the socially disaffected or the mentally unstable. Even when a potential terrorism suspect comes to the attention of U.S. law enforcement — as Mateen did — there may not be enough evidence, resources or coordination to continue an investigation.

“Law enforcement is following hundreds of people and there are thousands of people that have come on their radar,” Shawn Henry, a former FBI executive assistant director, said in an interview. “The complexity of trying to navigate our laws and Constitution while trying to maintain optimal security is a really difficult challenge. You just cannot protect against everything.”

The FBI said it interviewed Mateen in 2013 because the agency was told he had made inflammatory remarks about having terrorist ties and again in 2014 because of a connection to an American who went to fight with Islamic State.

“Those interviews turned out to be inconclusive, so there was nothing to keep the investigation going,” Ronald Hopper, an FBI assistant special agent in charge of the bureau’s Orlando office, told reporters Sunday.

Senior U.S. national security officials, including Federal Bureau of Investigation Director James Comey, have warned for several years about the threat from “lone-wolf” terrorists — those who become self-radicalized and plot attacks with little notice or resources.

The challenge for intelligence and law enforcement officials, however, is knowing when radical beliefs have crossed the line into action.

Speech, Gun Rights

“The step from just having some extreme views to acting violently is extremely hard to detect from the outside unless that individual is sharing that with others on social media, telephonically, or what have you,” said Daniel Benjamin, director of the Dickey Center at Dartmouth College and a former State Department counterterrorism coordinator.

That effort is compounded in a country like the U.S., where free speech and the ability to buy guns are considered fundamental rights. “Just making statements isn’t enough to arrest somebody,” Republican Senator Marco Rubio of Florida, a member of the Intelligence Committee, said on CBS News.

‘High-End Attacks’

“The ominous feeling today is that while lone-wolf operators may 99 percent of the time not be capable of complex, high-end attacks on their own, we’ve been reminded of how much damage an assault weapon can do,” said Benjamin.

Officials say it is too early to determine what, if any, actual link Mateen had with Islamic State, despite his 9-1-1 call and the group’s announcement claiming credit for the assault. Even as Islamic State has lost territory in Syria, Iraq and Libya, individuals connected or inspired by the group have carried out successful attacks in the past year in Paris, Brussels, San Bernardino, California — and now perhaps Orlando.

In a video posted on his Facebook page in the aftermath of the shootings, Mateen’s father Seddique Mateen, who in prior posts railed against both Pakistan and Afghanistan’s government, described his son as a good, educated boy who admired and respected his parents.

“I really don’t know what the motive was behind his act and I didn’t know he was so aggressive and angry about gays,” Mateen said. He blamed Pakistan for creating mental illness in Afghan youths, saying it had brought “sorrow, terrorism and deaths” to Afghanistan.

Ramadan Period

Representative Adam Schiff of California, the top Democrat on the House intelligence committee, said in a statement that Islamic State leaders have been urging attacks during the Muslim holy period of Ramadan, which started this month. It’s also LGBT Pride month in the U.S., raising additional questions about the attack as a hate crime against the club’s gay clientele.

FBI field offices around the country are constantly evaluating their surveillance operations and what investigations to prioritize, said Henry, the former FBI official who is now president of cybersecurity company CrowdStrike Services.

“The overwhelming numbers and the somewhat limited resources of the FBI do not allow them to track everybody all the time,” Henry said.

The attack also points to another troubling aspect in the fight against extremism.

Seizing Territory

Early on, Islamic State was viewed as less of a direct threat to the U.S. and Europe than other groups such as al-Qaeda because it was focused on seizing territory for its claimed caliphate. But Islamic State should be seen as both an insurgency — which is struggling to hold on to its territory — and as a global terrorist organization, which is recording successes, said Charlie Winter, a senior research associate at Georgia State University who studies transnational jihadist movements and insurgencies.

In terms of its ability to inspire attacks, Islamic State “looks more powerful than it has before,” Winter said in a phone interview from Atlanta.

One of the main unanswered questions now is whether the FBI informed local police officers in Florida so they could have kept tabs on Mateen, said Representative Peter King, a New York Republican who serves on the House Intelligence and Homeland Security committees.

“Often one incident by itself doesn’t mean anything,” King said. “I hope that it turns out the FBI did share that information with the local police. If not, then that would definitely be a problem.”

Coordination between the FBI and local police about suspects should have been learned from the Boston Marathon bombing in 2013, King said. Deceased bomber Tamerlan Tsarnaev came to the attention of the FBI before the attack, but that case was also closed and the information wasn’t shared with local authorities, King said.

Stopping lone-wolf attacks in the U.S. is “extremely difficult” because law enforcement and intelligence officials are limited by laws in terms of how much internet surveillance they can do of suspects, as well as by the resources they have, King said.

“In many ways it’s searching for a needle in the haystack,” King said. “The FBI doesn’t have the personnel to keep tabs on everybody.”