Dozens Under Watch in the US to Guard Against Paris Copycat Attack Officials are staying on top of the suspected radicals "like a blanket."

 -- Dozens of suspected radicals in the United States are under "tight surveillance" because law enforcement officials fear they might try to pull off a copycat version of the Paris attacks after being exposed to ISIS propaganda, according to FBI Director James Comey, during a rare joint briefing with U.S. Attorney General Loretta Lynch on Thursday.

The news came as senior law enforcement officials told ABC News Thursday that they are aware of a possible second ISIS video threatening Washington, DC. Authorities believe the video is part of an ISIS propaganda campaign to scare the American public in the wake of the Paris attacks. However, officials maintain that they have not identified a specific, credible plot targeting the U.S. homeland.

The suspected radicals have been under watch for months, but there are now fears that they might try to imitate last week's attack in Paris, which killed 129, Comey said.

"We are making sure we apply additional scrutiny to anybody who might be tempted to be a copycat, and so we’ve gone through all of our caseload to try and assess which are the ones that might pose a risk of trying to copy what happened in Paris and make sure that we’re staying on them like a blanket," Comey said.

He said that "hundreds" of FBI investigations have tried to determine where people consuming ISIS propaganda are on “the spectrum from consuming to acting."

Still, Lynch and Comey sought to reassure the American public today, telling reporters that they are not aware of any credible threats of a “Paris-type attack” to the homeland.

"We take all threats seriously. We're acting aggressively to diffuse threats as they emerge and we’re vigorously investigating and prosecuting those who seek to harm the American people," said Lynch.

Lynch pointed out that the FBI has arrested at least 70 people in the U.S. on charges related to violent extremism since 2013.

ISIS has threatened to attack New York and Washington, D.C., in new videos released this week, but the threats were not specific.

Comey said there was no evidence the Paris attackers had connections in the U.S. He said the biggest threats to the U.S. from ISIS were “troubled souls in America” who may be inspired by the militant group to cause harm and destruction.

“Counterterrorism is what you pay us to do," Comey said. “Tell us what you saw and then go on living your lives...that is channeling fear into something healthy, which is awareness of your surroundings, and not something disabling.”

He said that there are still about 900 open terrorism investigations in the U.S. When asked about the numbers of people that have gone to Syria from the U.S., he said there were 250 in total and of those, only "a subset" aligned themselves with ISIS. Of the subset, only a "teeny" number of people actually returned to the U.S., Comey said.

"We have a pretty good handle on that," he said.

Both officials addressed the increased level of anxiety that has arisen since the Paris attacks.

“Our highest priority will remain the safety of all Americans,” Lynch said. “We are operating around the clock to disrupt any plot.”