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By Erin McClam and Andrew Blankstein

The governor of New York put his state’s emergency operations center on higher alert Friday because of warnings by the federal government about a greater threat of terrorist attacks over the July Fourth holiday.

Gov. Andrew Cuomo said additional staff would be in place at the center. He also ordered state authorities to enhance monitoring of holiday celebrations.

He urged people to "not only remember the freedoms that we hold dear, but also remain cautious of their surroundings and learn to recognize and report suspicious activity."

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.”

And last week Homeland Security Secretary Jeh Johnson said that terrorist attacks in three countries on the same day were a reminder of the terror threat. He pledged vigilance by U.S. authorities, “particularly with the upcoming July Fourth holiday.”

Warnings about the threat of terrorism around major holidays have become routine since Sept. 11, 2001. And U.S. authorities have said there are no specific or credible threats to the United States. But the rise of ISIS and ISIS-inspired lone-wolf attacks over the past year has led to increased concern.

"Obviously a lone wolf is our worst nightmare, but we have nothing to indicate we have any of that going on," Commissioner William B. Evans of the Boston police told reporters Friday.

July Fourth falls this year during the Islamic holy month of Ramadan.

ISIS believes "that during the holy month of Ramadan, greater jihad or jihad against the infidels would be highly rewarded," Laith Alkhouri, an NBC News analyst and an official at the security consulting company Flashpoint, said Friday on MSNBC.

The July Fourth celebration at the largest U.S. Air Force base in Britain was scrapped because of “the most current local threat assessments.”

On Thursday, police and federal officials swarmed to the Washington Navy Yard, the site of a gun rampage two years ago that left 12 people dead, after a report of possible gunshots.

It turned out to be nothing, but city officials took the opportunity to encourage vigilance.

Here's a rundown of how some cities are preparing:

New York: The big event is the Macy's fireworks show on the East River. There is no known threat, but police said their "enhanced counterterrorism and security measures" will be greater this July Fourth than in the past. New York police stepped up patrols after the attacks last week in France, Kuwait and Tunisia.

Washington: A huge crowd will gather for fireworks and a free concert on the National Mall. Washington police stressed on Thursday that, after the scare at the Navy Yard, that they are always at heightened alert.

Boston: The main event is fireworks and a concert by the Boston Pops on the Charles River esplanade. Massachusetts State Police said uniformed troops, tactical units, and helicopters would be on patrol, along with an undercover officers and security cameras. It will be illegal to fly unmanned aircraft near the site of the celebration on Friday and Saturday.

Los Angeles: Several dozen fireworks shows are planned around Los Angeles. There are no known threats, but police are concerned about radicalized people who might try to target public gatherings. Police have increased security and staffed up. "Every LAPD officer out there this holiday, in addition to worrying about drunk drivers, illegal fireworks and other crimes, has to be vigilant," said Cmdr. Andrew Smith.

San Diego: Fireworks on San Diego Bay are among a dozen displays planned. Officials always have heightened concerns about San Diego because of its large military population.

San Francisco: The drought has cut the number of fireworks shows. Police will have extra officers on duty and are encouraging people to report anything suspicious. "Officers can’t be everywhere at once," said spokesman Albie Esparza. "We depend on our public to be our eyes and ears to report any suspicious activity."