Although no specific plot has been identified, revelers this Fourth of July can expect extraordinary security -- from expanded patrols and SWAT units to biological weapon response teams and undercover officers mingling with celebratory crowds.
"It's absolutely not a joke," said New York police Sgt. Thomas Horvath, who took ABC News on patrol around New York City's waterways. "The threat is very real. On the day of the [July 4] event, we're going to put out basically every asset we have."
And those assets will likely include a boat designed for the NYPD that detects radiation from a nuclear bomb or radiological "dirty" bomb.
Its sophisticated equipment monitors all radiation readings in the area. Any suspicious readings can be instantly sent to the Brookhaven laboratory in Washington D.C. for analysis by nuclear physicists.
There are only two such boats in the world.
July Fourth: 'Signature Holiday'
This year, police are especially concerned because evidence from Osama bin Laden's compound revealed that he wanted to kill Americans on July Fourth or by the 10th anniversary of Sept. 11.
"We all know that the Fourth of July is a signature holiday," Los Angeles Police Chief Charlie Beck said today. "Some of our enemies abroad would like to exploit that."
With tourism season going full tilt and this week's attack on a hotel in Afghanistan, police are boosting patrols in hotels too.
"I have deployed officers at a number of hotel locations around the city," New York Police Department Chief Ray Kelly said. "We are on alert. No question about it."
Kelly told ABC News that he'd been preparing for an attack similar to the one in Afghanistan and in Mumbai, India, where terrorists assaulted hotels for hours with small arms and weapons.
'Best Security Plan I've Ever Seen'
The U.S. Park Police will be on the case over the holiday weekend, too, said Teresa Chambers, chief of the U.S. Park Police, which oversees security on the National Mall.
The Park Police and FBI will have extra units and patrols on alert. Backpacks and coolers will be checked as people go through them looking for contraband, alcohol, fireworks and weapons.
"Within the United States Park Police, every tactical aspect that we have available and every uniformed person will be working," Chambers said. "What's important is what you won't see -- plainclothes officers, enhanced technology, a coordinated regional effort. It's the best security plan I've ever seen."
The FBI will be operating its command center at the FBI Washington field office, and the FBI's National Capital Response Squad, with units trained in crisis response, will be on the watch during the holiday weekend.
ABC News' Jason Ryan contributed to this story.