There's a key difference between the Orlando attack and past ISIS-claimed massacres

The terrorist group ISIS has claimed responsibility for another massacre - an attack on a gay nightclub in Orlando that killed at least 50 people.

The shooting was the deadliest in US history. The suspected gunman, 29-year-old Omar Saddiqui Mateen, reportedly pledged allegiance to ISIS (also known as the Islamic State, ISIL, or Daesh) in a 911 call.

After news outlets reported this, the ISIS-affiliated Amaq agency released a statement on its online propaganda channels claiming the attack.

But the statement differed from those released after other ISIS-claimed attacks in Paris and Brussels. In the Amaq statement released Sunday, the ISIS link to the Orlando attack was attributed to a "source." The brief statement also did not describe or provide any details about the attack.

While the Paris and Brussels attackers had direct ties to ISIS leaders, it's unclear how closely Mateen is connected to the group.

Michael Horowitz, a geopolitical and security analyst at the Levantine Group, a Middle-East based risk consultancy, told Business Insider that so far, there's nothing "that even remotely proves the attacker was in contact with ISIS."

He explained in an email:

The Amaq statement provides very little details regarding the attack, and even uses a relatively cautious phrasing by saying 'source to al-Amaq,' suggesting the group had no prior knowledge of the attack. The statement also refrains from directly saying that ISIS is responsible for the attack but rather indicates that 'an Islamic State fighter' carried out the attack. Both these elements suggest the attack was ISIS-inspired rather than directed or financed by the group.

Rita Katz, an expert on ISIS propaganda and co-founder of the SITE Intelligence group, made a similar assessment.

"There is no doubt that this message from Amaq is different than the claim after the Brussels attack," she told Business Insider in an email. "ISIS' Amaq message claim that shooter, Omar Matteen, is an ISIS fighter, seems to be based on the media reports that he pledged to ISIS."

The method in which the statement was released is also unusual. For past attacks, ISIS has released "official" statements that are directly from the group, rather than from the ISIS-linked Amaq, which acts as a news service for ISIS but is not officially part of the terrorist group's media wing.

"A direct statement from ISIS would have had more weight," Horowitz said.

"It is not uncommon for ISIS to release its first - and sometimes only - claim via al-Amaq, yet major 'operations' such as the Paris attack or the downing of the Russian plane [in Egypt] have been claimed through official ISIS statements first, and later an al-Amaq communique."

The social-media response from ISIS supporters has also been muted compared to past attacks.

After the Paris and Brussels attack, ISIS' online channels encouraged supporters to blast out canned messages on their social media accounts. Channels on the encrypted messaging app Telegram, which ISIS uses to send out information to supporters, asked followers to post canned messages celebrating the attacks and threatening more violence.

With the Orlando attack, there was no similar campaign.

"ISIS supporters praised the attack on social media, however, there is no overwhelming output from pro-ISIS media groups" like there was after the Paris and Brussels massacres, Katz said.

This further indicates that the Orlando shooting is likely to be a "lone wolf attack and was not coordinated with ISIS leadership as an ISIS operation," she noted.

The lone-wolf strategy

For ISIS, being inspired by the group's messaging is enough for their leadership to claim attacks as their own. Such was the case in the shooting in San Bernardino, California last year, which was carried out by ISIS supporters.

"With the ISIS accepting all who pledge to it, the Amaq report on the shooter being an IS fighter doesn't necessarily mean he coordinated with the IS prior to the attack, but acted in their name and they accept it as their own," Katz said.

ISIS has been encouraging so-called lone-wolf attacks as it loses ground in the Middle East. Much of the group's recruitment efforts are based on the message that ISIS is "remaining and expanding" - thousands of foreign fighters flocked to ISIS' de-facto capital of Raqqa, Syria when the group looked like an unstoppable force.

But that message has been damaged recently as ground forces backed by a US-led anti-ISIS coalition have succeeded in taking back territory from the terrorists. Thus, to maintain its powerful image, ISIS has started relying more on external attacks.

The group has gone from calling all Muslims to come to its self-declared "caliphate" in the Middle East to encouraging its supporters to remain in their home countries and commit attacks there.

Last month, ISIS spokesman Abu Muhammad al-Adnani released an audio message calling on ISIS supporters to mount attacks in Western countries.

"The smallest action you do in their heartland is better and more enduring to us than what you would if you were with us," Adnani said in the statement. "If one of you hoped to reach the Islamic State, we wish we were in your place to punish the Crusaders day and night."

Adnani also noted that some supporters are reluctant to kill civilians. He then provided justification.

"Know that inside the lands of the belligerent crusaders, there is no sanctity of blood and no existence of those called 'innocents,'" Adnani said. "... Know that your targeting those who are called 'civilians' is more beloved to us and more effective, as it is more harmful, painful, and a greater deterrent to them."

Horowitz noted that these messages can be effective as calls to action for radicalized people.

"These messages by ISIS's leadership are meant to maximize the psychological impact of these attacks among the Western public," he said.

"They create the perception that ISIS does control these attacks from within the 'safety' of its Caliphate in Iraq and Syria. In general, however, ISIS has little control over these radicalized individuals, other than pushing them to act within a specific timeframe."

And ISIS might have seen this latest attack as an opportunity to claim a success story on US soil.

"For the first time, the group seems to be 'taking a risk' by claiming an attack without being fully aware of the surroundings of the alleged 'pledge of allegiance,'" Horowitz said. "This may stem from the group's situation, as it faces multiple offensives in Iraq and Syria, and would also explain the phrasing 'source to al-Amaq' before the statement."

Horowitz noted on Twitter that ISIS haste in claiming the attack "shows just how much the group was waiting for it to boost its morale as it faces multiple offensives" in the Middle East.

Mateen was known to US law enforcement. He was on an FBI list of suspected ISIS sympathizers, and federal authorities had looked into him in 2013 and 2014, officials said Sunday.