Against a backdrop of horrific terror attacks abroad, the NYPD and other law enforcement officials are watching the approach of the July Fourth holiday with a sense of concern that "lone wolf" attackers could spark an incident here.

NYPD Commissioner William Bratton said Monday there is no known threat against the city for the holiday weekend -- capped with a spectacular fireworks display on the East River.

Bratton said the city had a "very elaborate plan as we normally have for crowd control but heightened in some respects for concern for acts of potential terrorism."

The police commissioner said that "what we are always worried about is not so much what we know but what it is we don't know."

Rep. Peter King (R-Seaford) said the recent attacks abroad and a number of local arrests of terror suspects in the metropolitan area is ramping up the worries of officials.

"I don't know of any period of time when we have had so many people arrested," King said yesterday referring to five local suspects taken into custody in recent weeks on charges of being involved with terrorist organizations, including the Islamic State.

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Those arrests, and the attacks in Tunisia, Kuwait and France last week, "raises the specter of a coordinated attack," King said. The Department of Homeland Security put out a routine bulletin cautioning law enforcement officials to be vigilant, but the tempo of events makes things different, King said.

The FBI has rounded up more potential lone-wolf terrorists, other federal officials have said, in response to the perception of the mounting threat of domestic attacks inspired by the Islamic State.

Since early May the Justice Department has announced the arrests of 10 people it says were inspired by and are supporting the Islamic State, also known as ISIS or ISIL.

The FBI has shifted its approach toward arrests rather than keeping suspects under surveillance, and it is targeting individuals thought to be planning attacks in the United States, federal officials said.

"Lately, we have seen an uptick in the number of arrests of ISIL followers who were planning violent acts in our homeland," said John Carlin, the assistant attorney general for national security. "ISIL . . . has demonstrated that they see value in mobilizing sympathizers anywhere in the world."

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Nassau police continue to "remain on heightened alert based upon the latest intelligence regarding terrorist threats directed towards law enforcement," Insp. Kenneth Lack, a spokesman for the department, said in an email. When asked what security precautions they were taking for the July Fourth weekend, Suffolk police officials said in an email they will hold a news conference Thursday "where we will be discussing all aspects of 4th of July Safety."

An NYPD spokesman wouldn't comment on specific action to beef up security for the holiday weekend.

But thousands of officers will be on duty for crowd control, officials said, deploying critical response vehicles at crucial locations and transportation hubs, setting up radiological and chemical detectors and using skywatch towers to monitor crowds.

But while police are monitoring known terror suspects, they're concerned about a lone actor under the radar.

"We are not worried about a big-scale event, but on the other hand all you need is one nut with a knife on 34th Street to start stabbing people," said a New York law enforcement official who didn't want to be named. "The new threat is not the known people but the unknown people."

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With Tania Lopez and Bloomberg News