Bomb disposal officers check for bombs at a Fort Pierce, Fla., apartment complex linked to the gunman in the fatal shootings at an Orlando nightclub on Sunday, June 12, 2016. (Alan Diaz/AP)

In its early expansion phase, the Islamic State called on Muslims across the world to join its ranks in Syria, urging not only fighters but also doctors and engineers to take part in what it billed as the historic restoration of the caliphate.

But the group’s most recent message essentially repealed that summons.

Don’t bother coming to Syria, Islamic State spokesman Abu Muhammad al-Adnani said in a recording issued May 21, because “the smallest action you do in their heartland is better and more enduring to us than what you would if you were with us.” As much as loyalists might still dream of reaching the caliphate’s borders, he said, those inside “wish we were in your place to punish the crusaders day and night.”

The bravado of the message masked the extent to which the Islamic State is being forced to revise its strategy after losing territory, cash and streams of recruits to opposition forces backed by U.S. airstrikes and Syrian government forces backed by Russian airstrikes.

But the devastation in Orlando represents a danger that many U.S. counterterrorism officials warn will be harder to contain than the Islamic State’s aspirations for an extremist haven in the Middle East.

FBI Director James Comey, right, listens to President Barack Obama speak to members of the media in the Oval Office of the White House in Washington June 13, 2016. (Pablo Martinez Monsivais/AP)

“The so-called lone-wolf attack . . . this is very difficult to deal with,” CIA Director John Brennan said in an interview with the al-Arabiya news organization just days before the shooting in Orlando. “Countries around the world are having to be concerned about the potential for individuals or groups of individuals to act on their own, without the direct contact with organized terrorists or groups.”

The shooting in Orlando is the deadliest example to date of the terrorism model Brennan described. The attack at the gay nightclub appears not to have been directed, funded or even approved in advance by the Islamic State’s operatives in Raqqa, its de facto capital in Syria.

As of Monday, U.S. officials said they still had seen no indication that Omar Mateen, the 29-year-old security guard accused of killing at least 49 people at the Pulse nightclub, had traveled to Syria or even had direct contact with members of the Islamic State.

But much like the attackers in San Bernardino, Calif., last year, Mateen inserted an extra step in his terrorist script to make sure the world would properly attribute his violence.

In one of three calls with 911 dispatchers after the attack was underway, Mateen declared his allegiance to Islamic State leader Abu Bakr al-Baghdadi — while also claiming solidarity with the Boston Marathon bombers and a Floridian who committed a suicide bombing in Syria for Jabhat al-Nusra, an al-Qaeda affiliate and Islamic State rival, according to James B. Comey, the FBI director, who briefed reporters Monday.

Comey, who refused to use Mateen’s name, said there were strong indications that “the killer” was inspired by foreign terrorist organizations, even if he didn’t always understand the distinctions between different groups.

FBI Director James B. Comey held a news conference at FBI Headquarters on the terror attacks in Orlando, Fla. (Reuters)

Baghdadi is seen as a sullen figure who lacks the charisma of Osama bin Laden or al-Qaeda’s flair for the dramatic. He has endorsed a more prosaic brand of terrorism that emphasizes hitting vulnerable targets and exploiting Western weaknesses.

As a result, counterterrorism experts believe that the Islamic State may only be in the early stages of demonstrating its ability to incite a kind of violence that doesn’t require penetrating the post-Sept. 11, 2001, defenses of the United States.

“Baghdadi understands that if he calls for terror, it will come,” said Bruce Riedel, a former CIA analyst who is a terrorism expert at the Brookings Institution. Baghdadi “doesn’t need any direct human connection or even a Web connection. His message is so pervasive in the media and so simple it is certain to inspire the angry. Add the easy availability of military-style weapons and we have a challenge that won’t die with Baghdadi.”

The threat is even greater in Europe, where security services must contend not only with lone-wolf actors but also with hundreds of experienced fighters who have returned from Syria with contacts and training — like those who carried out the multistage attacks in Paris and Brussels.

For several years, U.S. officials have quietly expressed relief that the nation’s greater distance from the Middle East and the massive spending on counterterrorism measures over the past 15 years serve as major barriers to plots like those in Paris or Brussels.

Orlando, however, exposed the limits of that protection.

Mateen’s anger also appears to have been driven by a deep-
seated animosity toward gays, a mind-set that would have melded seamlessly with the online rantings of a terrorist group so violently homophobic that it has posted videos of accused homosexuals being thrown from rooftops to their deaths.

President Obama said Monday that while “there’s no direct evidence that he was part of a larger plot,” Mateen “was inspired by various extremist information that was disseminated over the Internet.”

The depth of Mateen’s religious convictions may not matter to the Islamic State, which claimed responsibility for the shooting and declared him a martyr to the group’s cause. Experts said that it is almost inevitable that the Islamic State’s bleak message has already drawn other silent recruits in the United States and that they will be emboldened by the nation’s inability to defend against mass shootings.

Although Mateen’s ties with the Islamic State, if any, are not fully understood, his actions and background are remarkably similar to those of other domestic terrorists involved in mass shootings in recent years. Even Mateen’s age, 29, is exactly the average for perpetrators in past incidents, said author Peter Bergen, whose book, “United States of Jihad,” looked at 300 U.S. cases.

“They tend to be well educated to at least an average education, and they tend not to have a criminal background or mental derangement,” said Bergen, vice president of New America, a Washington think tank. “Typically they are American citizens. And without exception, none of them have been part of foreign terrorist organizations. They may have been inspired, but they’re not working with them.”

Based on what is known of Mateen, Bergen said, “this guy already fits that profile very well.”

Joby Warrick contributed to this report.