WASHINGTON • Since the Sept 11 attacks in 2001, United States counter-terrorism 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 US is more susceptible now because of tactics chosen for recent terrorist strikes by the Islamic State in Iraq and Syria (ISIS) militant group.
Officials cite an increased effort by ISIS to galvanise its sympathisers in the US and elsewhere since Memorial Day and during this Ramadan season to carry out acts of violence on their own - so-called lone-wolf attacks.
"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 US," said Mr Matthew Olsen, a former director of the National Counterterrorism Centre.
"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," he also said.
On Thursday morning, the concerns of senior US officials were heightened after gunshots were reported at the Washington Navy Yard, where 12 people were killed in a shooting in 2013.
Hundreds of law enforcement officers descended on the navy yard on Thursday, but found no evidence that there had been any shooting.
Officials have blamed the ISIS militant organisation for inspiring last week's bloody strikes in France and Tunisia, and possibly in Kuwait.
"This isn't like Al-Qaeda, where there were networks and large- scale plots and well-trained guys," said a 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're always concerned going into holidays like the Fourth of July or Memorial Day that deal with patriotism," said a senior US intelligence official who, like others, spoke on condition of anonymity to discuss confidential assessments. "But there's more concern than there was last year."
In particular, US officials say, ISIS has told its followers to attack law enforcement and uniformed military personnel, many of whom appear prominently in public settings on July 4.
NEW YORK TIMES