Omar Mateen pledged his allegiance to ISIS, but in the aftermath of the Orlando massacre, familiar questions are being asked: was he directed to kill by leaders of the self-proclaimed Islamic State, or did he invoke the ISIS name to add notoriety to what would become the worst mass shooting by a single shooter in American history?

Al-Bayan, the ISIS radio station that broadcasts in Iraq, claimed Monday morning that Mateen was "one of the soldiers of the caliphate in America."

A short statement on Sunday from the ISIS-linked Amaq News Agency claimed "an Islamic State fighter" was responsible for the killings, but the brief statement, short on details, offers a hint that ISIS leaders were unaware of the attack before it happened.

"I would be very surprised to find out that they had known him in advance," said Yoram Schweitzer, who studies terrorist organizations at the Institute for National Security Studies in Tel Aviv, Israel.

With the investigation into Mateen is still in early days, law enforcement agencies have yet to find any evidence that the 29-year-old American had direct links to ISIS. It appears his radicalization happened recently.

Little evidence of networks

Mateen's call to a 911 operator pledging allegiance to ISIS is the third time an attacker in the United States has dedicated an act of violence to the jihadist group.

Tashfeen Malik posted an oath of loyalty to ISIS on her Facebook page before she and her husband, Syed Rizwan Farook, killed 14 people in San Bernardino, Calif., last December.

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Mateen's ex-wife, Sitora Yusufiy, told media in Boulder, Colo., that Mateen was 'not a stable person' and had beaten her when they were married. The two divorced in 2011. (Autumn Parry/Boulder Daily Camera/Associated Press)

"The investigation so far has developed indications of radicalization by the killers, and of potential inspiration by foreign terrorist organizations," said James Comey, the director of the FBI, said at the time. "There's no indication that they are part of a network."

One of two gunmen who tried to ambush an event in Texas featuring controversial cartoons of the Prophet Muhammad, Elton Simpson, used Twitter to link himself to ISIS. Along with Nadir Soofi, Simpson was killed by police in Garland, Texas, in May 2015.

Since its inception, ISIS has used its vast propaganda network to call on fighters around the globe to carry out attacks in its name.

It did just that about three weeks ago, when the group's spokesman, Abu Muhammad al-Adnani, urged followers to commit acts of violence in a speech ahead of the start of the Muslim holy month of Ramadan, which began last week.

"We have a special message to the soldiers of the caliphate and its supporters in Europe and the U.S.," Adnani said "The smallest attack you carry out on their own turf is better and more beloved to us than the largest attack we carry out here. It is more useful to us and more harmful to them."

ISIS happy to claim 'lone wolf' attackers as its own

It was Adnani who called for attacks against Canadians in September 2014. The spokesman also praised Michael Zehaf-Bibeau for killing Cpl. Nathan Cirillo at the National War Memorial in Ottawa a month later. Adnani said that followers who had "the opportunity to shed blood" should do so.

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Forces loyal to Syria's President Bashar al-Assad flash victory signs and carry a Syrian national flag on the edge of the historic city of Palmyra in Homs in March. ISIS has suffered several defeats in Syria and Iraq recently, and its attempt to claim responsibility for the Orlando massacre might be a way to divert attention from its losses. (Handout/Sana/Reuters)

Despite these acts of violence being interpreted by many analysts as the actions of "lone wolf" attackers, ISIS is keen to claim them as its own. It gives the militants publicity, and in the case of the Orlando massacre, draws attention away from the group's recent losses.

ISIS fighters are on the defensive in Fallujah, Iraq, and in an area near its de facto capital, Raqqa, Syria. An analysis by the respected defence and security consulting group IHS, which owns the Jane's publishing group, found that ISIS has lost more than 20 per cent of its territory in Iraq and Syria.

"ISIS is not denying [the Orlando attack] and definitely would like to embrace it," Schweitzer said. "The attack increases their power image, especially in this time of dire straights, as they are in now."