The Orlando nightclub killings mark the third time in just over a year an attacker has claimed allegiance to the Islamic State, a group that has aggressively advocated for such lone wolf attacks.
So how significant is this?
One school of thought is that individual shooters are simply seeking maximum publicity by invoking the Islamic State, yet have no real links, and therefore the claim is of limited value in understanding the motive or preventing future attacks.
A countervailing view is that the Islamic State's extremist ideology is inspiring shootings that are likely to continue as long as ISIS is seen as a powerful force. Therefore, the argument goes, the Islamic State needs to be routed as quickly as possible to strip the group of its appeal for radicals.
Omar Mateen, the man blamed for the Orlando shooting, pledged his allegiance to ISIS in 911 calls in the early stages of the attack Sunday morning. He targeted the LGBT community, which the Islamic State has dealt with in the most brutal fashion, throwing suspected gay men to their deaths from rooftops in Syria.
The FBI said it questioned Mateen in 2013 and again in 2014 over possible links to terrorist groups, but found no compelling evidence after an inquiry that lasted nearly a year. FBI Director James Comey said Monday there were "strong indications of radicalization by this killer," but no evidence that he was acting on orders by ISIS or any other group.
Similar Pledges
The investigation is still in the early stages, though Mateen's pledge to ISIS has echoes in two earlier cases.
Tashfeen Malik declared her allegiance to ISIS in a Facebook post last December as she and her husband launched a shooting spree that left 14 dead in San Bernardino, Calif. The FBI and other law enforcement agencies have investigated, but haven't announced any ties between the couple and ISIS.
And in May 2015, Elton Simpson directed his Twitter followers to an ISIS supporter who tweeted on his behalf as he opened fire with another man in a Texas parking lot outside a Prophet Muhammad cartoon contest. Simpson and his fellow gunman were both killed.
In all three cases, the Islamic State claimed credit after the pledges by the shooters were publicized.
Analysts say the pledges are troubling, but it's difficult to determine what role, if any, ISIS played in inspiring these attacks.
"These individuals were already radicalized before the Islamic State was on the radar screen," said Daniel Byman, who tracks terrorism at the Brookings Institution. "But it's reasonable to assume that Islamic State propaganda has some impact. Groups do this for a reason. And from the Islamic State's point of view, as they have more failures in Iraq and Syria, they will seek more successes with attacks abroad."
"What I don't know is whether the propaganda is a determining factor when it comes to carrying out an attack," Byman added.
A Call To Target Westerners
After the U.S. and its allies began bombing ISIS in August 2014, the ISIS online magazine Dabiq urged supporters to kill Westerners wherever they could. Here's an excerpt from it's October 2014 edition:
"At this point of the crusade against the Islamic State, it is very important that attacks take place in every country that has entered into the alliance against the Islamic State, especially the U.S., U.K., France, Australia, and Germany. Rather, the citizens of crusader nations should be targeted wherever they can be found."
Since then, ISIS has been linked to attacks in France, Belgium and the U.S. But analysts say it's important to distinguish between attacks directed by ISIS and those in which the attacker may have acted alone.
The mass killings in Paris last November and in Brussels in March were coordinated by multiple attackers with direct ties to the extremist group, according to authorities in France and Belgium.
In the U.S., the links are much fuzzier, though terrorism immediately became the focus of the presidential campaign on Monday.
"The threat is metastasizing," the presumptive Democratic nominee, Hillary Clinton, said in Cleveland. "We saw this in Paris, and we saw it in Brussels. We face a twisted ideology and poisoned psychology that inspires the so-called lone wolves: radicalized individuals who may or may not have contact and direction from any formal organization."
Republican Donald Trump, meanwhile, criticized President Obama for not describing the Orlando shooting as "radical Islamic terrorism."
"Unless you know the words and unless you know what's going on, you're never going to solve the problem," Trump said on NBC's Today show.
The U.S. bombing campaign against ISIS has contributed to its steady loss of territory in both Syria and Iraq. However, some critics have called for a stepped-up campaign, saying the current one is not sufficient to fully defeat ISIS.
Greg Myre is the international editor of NPR.org. Follow him @gregmyre1.
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