WASHINGTON » Since the attacks of Sept. 11, 2001, U.S. counterterrorism officials have increased security at national landmarks and large public events ahead of major holidays or significant anniversaries, and issued bulletins calling for increased vigilance by the public.

But this Fourth of July weekend has spawned particularly strong warnings about a potential attack, as the federal authorities and national security experts say the United States is more susceptible now because of tactics chosen for recent terrorist strikes by the Islamic State.

Officials cite an increased effort by the Islamic State to galvanize its sympathizers in the United States and elsewhere since Memorial Day and during this Ramadan season to carry out acts of violence on their own — "lone-wolf" attacks. Those potential plots by individuals are harder for the authorities to detect and disrupt, senior U.S. officials say, and have led the FBI to put more Americans under investigation for suspected ties to terrorist groups than at any point since Sept. 11.

At the same time, senior U.S. officials acknowledge that, as with many past holidays and anniversaries, they know of no specific terror plot against targets in the United States. And while militants have said in previous years that they would like to pull off an attack on a major national holiday, there has not been a successful terrorist strike on U.S. soil on a previous July 4 or similarly high-profile day.

"We’ve been through other periods of high threat, but this time is different because of the proven reach of ISIS into Europe and the U.S.," said Matthew G. Olsen, a former director of the National Counterterrorism Center, using another name for the Islamic State. "Even in the absence of a specific plot, there’s reason for alarm given the number of people who seem to have been motivated by ISIS’ rhetoric and direction to carry out attacks against innocent people."

On Thursday morning, the concerns of senior U.S. officials were heightened after gunshots were reported at the Navy Yard in Washington, where 12 people were killed in a shooting in 2013. Hundreds of law enforcement officers descended on the Navy Yard on Thursday, but they found no evidence that there had been any shooting.

The Islamic State, also known as ISIS or ISIL, has shown success at inspiring rather than actually directing well-planned attacks — and officials blamed the militant organization for inspiring last week’s bloody strikes in France and Tunisia, and possibly in Kuwait. That approach by the Islamic State has shaken the confidence of U.S. officials in their intelligence, as those inspired by the Islamic State have proved more likely to act quicker than al-Qaida militants, who communicated for several months with leaders in Afghanistan and Pakistan as they planned attacks.

"This isn’t like al-Qaida, where there were networks and large-scale plots and well-trained guys," said one senior federal law enforcement official. "ISIS is telling these guys: ‘Do anything you can. Go to an Army-Navy store and buy a knife and stab someone. Just do something.’" The official noted, however, that "the pool is much bigger than it’s ever been but it’s not nearly as deep."

"We have a lot of guys who don’t have connections to others in the U.S. who have shown some interest online in ISIS, but it’s unclear how serious they are so we’ve had to keep on them," the official said.

Intelligence officials have seen at least one Twitter message of an ISIS supporter citing the U.S. government’s warnings for this holiday weekend — apparently in boastful terms. While officials say they do not know of any specific threat, they also know they will be the first ones criticized by the public and members of Congress if there is an outbreak of violence.

"We’re always concerned going into holidays like the Fourth of July or Memorial Day that deal with patriotism," said a senior U.S. intelligence official who, like others, spoke on the condition of anonymity to discuss confidential assessments. "But there’s more concern than there was last year."

In particular, U.S. officials say, ISIS has told its followers to attack law enforcement and uniformed military personnel, many of whom are prominent in public settings on July 4.

The attention the Islamic State has received in the past year has led politicians and the news media to use the holiday as a way to reinforce their views that the group is one of the greatest threats to the United States, and to criticize the Obama administration.

In a lengthy news release that included charts and graphs, the chairman of the House Homeland Security Committee, Rep. Michael McCaul, R-Texas, said the time had come for President Barack Obama "to listen to these wake-up calls and to lay out a credible offensive strategy to defeat what is a global — not regional — menace."

"I commend our law enforcement and intelligence professionals for disrupting so many plots recently, but we cannot rely on our defenses alone," McCaul said. "Extremists are radicalizing and recruiting new foot soldiers around the world at broadband speeds, yet the administration is responding at bureaucratic pace."

A chart in the release showed that there had been 28 ISIS-linked plots against Western targets in 2015 compared with 19 in 2014. Another chart said the arrests of 55 people with ties to ISIS in the United States this year was a fivefold increase from last year.

Speaking to reporters Wednesday, Defense Secretary Ash Carter discussed the terrorism risk, saying that lone-wolf attackers inspired by the Islamic State posed possible threats to security over the coming holiday.

"In the era of social media, a phenomenon like ISIL, unlike al-Qaida of the old days, there doesn’t have to be and won’t necessarily be a command-and-control relationship between somebody who instigates an incident and ISIL as an organization," Carter said at a Pentagon briefing.

"They are self-radicalized, self-organized people on social media," Carter said. "Are we concerned about that? Absolutely, we’re concerned about it. And that has been seen in Europe, so that’s a sign that in the United States, as in Europe and other places in the world, we have to be concerned about it, and we obviously are."

Independent analysts who follow jihadi statements on the Internet said they have not detected any significant increase in online comments — what the authorities called "chatter" — but acknowledged that lone attackers inspired by ISIS leaders and propaganda could go undetected.

"There isn’t much chatter on jihadi online platforms regarding specific threats to the homeland over the holiday weekend, nor am I seeing any response so far to the U.S. government’s warning," said Laith Alkhouri, a senior analyst at Flashpoint Global Partners, a security consulting firm that tracks militant websites.

"However, ISIS fighters on the ground have increasingly preached attacks in the U.S. and England, releasing bomb-making instructions and multiple threats," Alkhouri said, adding that they advocated for attacking parades and other events.

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