Counterterror investigators in the United States are stepping up monitoring of jihadi sympathizers on websites linked to the Islamic State group, focusing on Internet-savvy Muslims with passports and a demonstrated interest in joining the terror group, according to law enforcement officials.

The push to identify and locate potential Islamic State sympathizers before they travel to Syria or Iraq -- where the group, also known as ISIS or ISIL, controls large swaths of territory -- has reached "fever pitch" in recent weeks inside the FBI and NYPD, said a federal official with knowledge of the investigative efforts.

"Their reach would appear to be growing in terms of how they're able to reach out directly to those receptive to their message in the U.S., Europe and elsewhere," the official, who spoke on condition of anonymity because he was not authorized to publicly discuss the issue, said of ISIS.

The number of websites and social media hosting forums and discussions sympathetic to the group's message, the official said, is "growing exponentially."

The digital reach of ISIS was highlighted Wednesday when three New York City men were charged with conspiring to aid the group and accused of musing about terrorist acts ranging from the murder of FBI agents or cops to killing President Barack Obama.

Two of the accused were arrested attempting to join the Islamic State group -- one caught at Kennedy Airport heading to Syria, authorities said. The other suspect, who allegedly helped the others financially, was arrested in Florida.

FBI Director James Comey, speaking Wednesday at a meeting of the National Association of Attorneys General in Washington, D.C., said his agency had investigations ongoing into suspected Islamic State supporters in all 50 states.

Comey said the group's message "resonates with troubled souls, people seeking meaning in some horribly misguided way. . . . Those people exist in every state."

State Department spokeswoman Jen Psaki recently said on CNN that the government was "combating" 90,000 ISIS-related propaganda messages on Twitter every day.

Intelligence officials said they believe that more than 150 American citizens have either traveled or tried to travel to Syria to join ISIS, Nicholas Rasmussen, director of the federal National Counterterrorism Center, told a congressional hearing earlier this month.

Most made the trek after seeing the terror group's videos, social media messages and other online propaganda, the federal source said.

"That's their strength," he said of ISIS' social media presence.

The FBI and NYPD said Thursday they would not discuss online investigative methods or surveillance used against suspected jihadists in America, citing ongoing casework.

"Our officers on the street have their eyes open, but the best information . . . and leads are usually going to come from the public," said an NYPD official involved in counterterror efforts, who spoke on condition of anonymity. "They are the ones who are going to see the signs of radicalization of social media. And we need them to pick up the phone when they do."