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By Halimah Abdullah

For two hours Thursday morning, it looked like the dire warnings of July 4th terror were coming true in Washington.

Police and ambulances swarmed the Washington Navy Yard after a worker said she might have heard gunshots. It turned out to be nothing, but authorities were quick to praise the woman for reporting possible trouble.

And law enforcement experts say it amounts to a dress rehearsal for the holiday weekend — a celebration of freedom that could be fraught with skittishness, especially in Washington.

“Any incident that you see in our nation’s capital or you see in any major city across the country will frankly receive a similar response this weekend,” said Clint Van Zandt, a former FBI profiler and an NBC News analyst.

American law enforcement is particularly concerned with the terrorist group's aggressive recruiting through social media and 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 as well as threat of violence specifically targeting police and military personnel, could be tempting, officials say.

There is additional urgency because of the symbolism of the holiday and events around the globe, most recently the series of attacks that killed 39 people in France, Tunisia and Kuwait.

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.

Authorities told NBC News that they are unaware of any specific or credible threat inside the country.

But law enforcement — especially those in large cities — are on the alert.

In New York, where people will turn toward the East River to view Macy’s annual Fourth of July fireworks show, "enhanced counterterrorism and security measures" will be greater than in past years. The New York Police Department was already stepping up patrols after last week's deadly terror attacks in France, Tunisia and Kuwait.

In Boston, where a fatal bombing two years ago at the city's famed marathon brought issues of national security home, Massachusetts State Police say uniformed troops, tactical units, and helicopters will be patrolling, along with an undercover presence and security camera footage at a large fireworks celebration over the Charles River.

Chicago, Los Angeles, San Diego and San Francisco have all also heightened their security presence in advance of the holiday.

And in Washington D.C., where an annual free concert and fireworks on the National Mall will take place, Thursday's scare highlights why officials say they must remain vigilant.

Still, the response to the reports of gunshots Thursday was warranted, experts said.

“When we talk about ISIS inspired actors, the FBI has been rolling up some of those guys in the last two weeks — some of them are lone wolves, but some of them have a brother, some of them have a friend, some of them have a couple associates that could link…and you can get some multiple inspired actors,” Jim Cavanaugh, a law enforcement analyst told MSNBC. “…You need a strong response so they can deal with it.”

However, there is good reason for law enforcement’s increased caution given the heightened concerns during the holiday weekend.

“This entire weekend they're going to have to take a high posture in Washington, D.C. And any incident that you see in our nation’s capital or you see in any major city across the country will frankly receive a similar response this weekend,” Van Zandt told MSNBC on Thursday.

The Washington Navy Yard in particular is a place where memories of just such an attack are still fresh.

On Sept. 16, 2013, Aaron Alexis, a former Navy reservist with a troubled mental health history, opened fire in an office building at the Navy Yard, paralyzing the capital before he was taken out in a gunfight with authorities.

Alexis was in the building for more than an hour before he was stopped. It was the second-deadliest rampage on an American military base, after the Fort Hood shootings in Texas in 2009. The building where Alexis opened fire reopened in February after renovations.

Chief Cathy Lanier of the Washington police said the response reflects the fact that the city’s law enforcement “never lower our threat level.”

Erin McClam, Tom Winter, Andrew Blankstein and Brittany Elena Morris contributed.