Late Tuesday, the federal government said there is "no specific, credible intelligence" of any plots against the United States. But law enforcement is very concerned about what it doesn't know.
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Within hours of the attacks in Brussels, the FBI stepped up surveillance of potential terrorism suspects in the U.S.
"We're gonna ramp up on all of our subjects here. Twenty-four/seven surveillance, and cover them just in the event that there is some type of connection," said Diego Rodriguez, an assistant director of the FBI.
"And if there is, then at least we will be already on them."
Officials are concerned about the potential for ISIS sympathizers to plot attacks in the U.S. There was a similar fear after the Paris attacks in November.
Frank Cilluffo, a former White House counter terrorism official, said one concern is the potential for copy cats in the U.S.
"You always need to be worried about copy cats because there's an old saying: terrorists are not innovators, they're imitators."
According to a report by the House Homeland Security Committee, more than one-third of ISIS-linked terror plots have been aimed at the United States or its interests overseas.
U.S. officials also estimate 30,000 foreign fighters have traveled to Syria and Iraq to join ISIS, including around 150 from the U.S.
One intelligence consulting group believes about 40 have returned -- those are the ones U.S. law enforcement officials are aware of.
But just last week, American Mohammad Jamal Khweis surrendered to Kurdish forces in northern Iraq. He had traveled from the Washington, D.C. region to Syria to join ISIS, and he was not on the FBI's radar.
"What you are seeing right now is we are shaking every known bush. You're going to have a number that won't necessarily be on anyone's radar screen and that is something we need to be cognizant of," said Cilluffo.
The couple behind December's San Bernardino attack was not on the FBI's radar, either. On Tuesday, Homeland Security sent a bulletin to law enforcement agencies across the country warning of the potential threats from ISIS.