FBI Director James Comey said Thursday that there is no credible threat to the United States following the Paris terror attacks, and that investigators have found no links between the attackers and the U.S.

Appearing before reporters Thursday alongside Attorney General Loretta Lynch, Comey said he was aware that the Islamic State group and its supporters regularly issue propaganda in videos and magazines boasting about their plans and capabilities. But, he said, "that is not credible intelligence."

The FBI is tracking dozens of Islamic State supporters in the U.S. whom agents are concerned could be prone to violence, and in response to the Paris terror attacks, are giving extra scrutiny to individuals who "might be tempted to be a copycat," he said.

"The risk with a troubled soul who is consuming the propaganda is that they might try and copy that which they saw in Europe," Comey said.

Separately, Lynch said the Justice Department opposes congressional legislation that would put up barriers to Syrian refugees seeking to enter the United States and would oblige the heads of the FBI and Homeland Security Department and the director of national intelligence to certify to Congress that each refugee "is not a threat to the security of the United States."

"From a law enforcement perspective, the bill presents us with an impracticality and impossibility," Lynch said. "To ask me to have my FBI director or other members of the administration make personal guarantees would effectively grind the program to a halt, and would essentially not provide the safety and security that I really think is the concern of everyone looking at this issue."

The attacks in Paris have stoked global fears about the flow of foreign fighters from Western nations in and out of Syria. U.S. officials are aware of roughly 250 people who have traveled or attempted to travel there from the United States, Comey said, although the number has slowed considerably in recent months for reasons that aren't clear.

"My hope is that the message has gotten out that it's hell on earth in the so-called caliphate," Comey said. But it could also be that IS has been urging its followers not to travel and to instead attack targets where they are, he said.

He described IS as a "death cult," not unlike the hundreds who drank poison at Jonestown, the commune in Guyana that was the scene of mass suicide in 1978. But, he conceded, they are very effective at using social media to send their propaganda of hatred to disaffected people all over the world, especially the young.

"They are broadcasting a message that is buzzing 24 hours a day in someone's pocket," Comey said. "That is a message of 'ultimate meaning.' That you can participate in the final battle between good and evil, you can find a source of meaning in your life that is transcendent. And that's a message that resonates with people that are unmoored. We are a country of over 300 million people. We have some unmoored folks."