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By Jon Schuppe and Andrew Blankstein

July 4 security alerts have become almost routine, with authorities warning every year that the holiday could inspire an attack by foreign or home-grown terrorists.

But the vigilance surrounding this year's celebration carries added weight.

The reason is ISIS.

The year-long rise of the radical Islam terror group, and its leaders' appeals for supporters to carry out attacks around the world, has prompted a sharper sense of caution.

American authorities are particularly concerned with an ISIS spokesman's urging of supporters to attack during the holy month of Ramadan, which ends July 17. The symbolism of American Independence Day, and mass gatherings of people at fireworks displays in large cities, could be tempting, officials say.

Federal intelligence agencies issued a bulletin to local law enforcement in May reminding them "to remain vigilant during upcoming national holidays and military events due to the heightened threat of attacks" by ISIS and its supporters.

U.S. intelligence and law enforcement authorities share a growing concern of an ISIS-inspired attack in the country through the July 4 weekend.Mary Altaffer / AP

Authorities told NBC News that they are unaware of any specific or credible threat inside the country. But the dangers are more complex and unpredictable than ever. And there is mounting evidence that many of those ISIS supporters, including so-called lone wolves who have been inspired if not explicitly backed by the group, are heeding those calls.

"This year is different from other years because of the real links that come across between ISIS' fighters on the ground connecting with radicalized individuals or self-radicalized individuals online in the homeland," said Laith Alkhouri, a co-founder of security consulting firm Flashpoint Intelligence, an NBC News partner.

"Al Qaeda has been pushing the lone wolf approach for a decade, with very modest results. ISIS has been much more successful with the tactic, in part because their recruiting pitch is much less sophisticated"

He pointed to the Boston case of Usaamah Abdullah Rahim, who, according to authorities, became fascinated with ISIS and was plotting to behead police officers. He was shot to death in a confrontation with police and FBI agents last month.

That followed a thwarted attack in May, when two gunmen were killed during an attempted assault on a "Draw Muhammad" contest in Texas.

FBI Director James Comey said the case illustrated the new landscape of terror, in which ISIS is helping to recruit and instruct aspiring terrorists around the world, including America, through social media and other online methods, including encrypted networks.

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To try to get ahead of the problem, the FBI has shifted its focus from tracking radicalized Americans who want to fight with ISIS abroad to those who stay home to plot their own attacks. Rather than put these suspected militants under long-term surveillance, the FBI is making quick arrests. Fordham Law School's Center on National Security studied the arrests and found that since late March there has been "a substantial increase in cases involving individuals accused of plotting attacks in the United States in the name of ISIS."

In many cases, the targets of those investigations don't seem to be part of an organized network, and their alleged plots appear decidedly low-tech.

It might seem easy to dismiss many of those arrested as amateurs, said Patrick Skinner, director of special projects for the Soufan Group, a security consulting company. "But the problem is that the FBI has so many of these individuals, thousands of them across the country, so they can’t take the chance that they're going to slip up and lose a guy and end up with a terrorist attack."

In the New York region alone, authorities have arrested five people on terror-related charges in the past month, including several who were allegedly involved in plots to help ISIS or to carry out an ISIS-inspired attack.

"We have no specific information about anything directed at this city, but as reflected by the arrests that have been made in this locale in the last couple of weeks, what we’re always worried about is not so much what we know but what we don’t know," New York Police Commissioner William Bratton said Monday.

J.M. Berger, who analyzes extremism at the Brookings Institution, said he believed there was a greater risk this year than in previous years.

"More broadly, while we have seen stepped up enforcement against ISIS supporters, it's impossible to detect every threat, especially when the perpetrators are minimally connected to a network," Berger told NBC News in an email.

"Al Qaeda has been pushing the lone wolf approach for a decade, with very modest results. ISIS has been much more successful with the tactic, in part because their recruiting pitch is much less sophisticated and more tuned to people who might already been inclined toward violence."