The chief of the FBI says ISIS now poses a greater threat to the United States than its plodding rival al Qaeda thanks to the tech-savvy extremist group’s ability to inspire troubled Americans to violence against their own country on social media.

Speaking to an audience at the Aspen Security Forum in Colorado Wednesday, FBI Director James Comey said that ISIS, which has proclaimed a caliphate in parts of Syria and Iraq, has influenced a significant but unknown number of Americans through a robust, year-long campaign on social media urging Muslims who can't travel to the Middle East to ‘kill where you are.’

Twitter handles affiliated with the group have more than 21,000 English-language followers worldwide, he said, thousands of whom may be U.S. residents.

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Sounding the alarm: Speaking to an audience at the Aspen Security Forum in Colorado Wednesday, FBI Director James Comey said that ISIS has influenced a significant number of Muslim Americans who can't travel to the Middle East to ‘kill where you are’

New kids on the block: Twitter handles affiliated with ISIS have more than 21,000 English-language followers worldwide who can communicate with members of the group at any point in time

Inspiring troubled souls: Comey said ISIS is trying to reach people that al Qaeda would never use as operatives: emotionally unstable, troubled drug users

The FBI has arrested a significant number of people over the last eight weeks who had been radicalized, Comey said, without specifying a number.

He repeated his previous disclosure, without elaborating, that several people were arrested who were planning attacks related to the July Fourth holiday.

The bureau has hundreds of investigations pending into such cases across the country.

Comey said it was too soon to say how Muhammad Youssef Abdulazeez, the Chattanooga gunman who killed four Marines and one US Navy sailor last week, became radicalized.

Abdulazeez's relatives have said he had a history of drug use and depression. Comey noted that ‘the people the Islamic State is trying to reach are people that al Qaeda would never use as an operative, because they are often unstable, troubled drug users.’

Asked if the threat from the Islamic State group had eclipsed that of the rival organization that attacked the US on September 11, 2001, Comey replied, ‘Yes.’

Old news: Al Qaeda's followers still have to rely on email to communicate to the terrorist organization once headed by Osama bin Laden (pictured)

Comey explained that while al Qaeda's followers still have to rely on email to communicate to the terrorist organization, ISIS is readily accessible all day long through Twitter, reported Aspen Times.

'If you want to talk to a terrorist, they’re right there on Twitter Direct Messaging for you to communicate with,' Comey said.

Comey said the threat from the Khorasan Group, al Qaeda's offshoot in Syria, significantly diminished after its leader, Muhsin al-Fadhli, was killed in an airstrike this month

The US has tracked dozens of Americans, ranging in age from 18 to 62, who have traveled to Syria or Iraq to fight with the Islamic State group, he said.

‘I worry very much about what I can't see,’ Comey added, because he said Islamic State group recruiters use encrypted communication software and human couriers to avoid US eavesdropping.

Comey has sounded the alarm about domestic radicalization before, but his remarks Wednesday signal a deepening concern among high-level officials within the government about the impact of ISIS’ effort to inspire terrorist violence.

As recently as September, senior US intelligence officials were downplaying the group's capacity to launch an attack on American soil.

Matt Olsen, then the head of the National Counter Terrorism Center, told Congress last fall that the US had ‘no credible information’ that ISIS is planning a strike against the US.

Intelligence officials last year were saying they worry most about a mass casualty attack against a US airliner by al-Qaida's Yemen affiliate, or by the Khorasan Group, a cadre of al Qaeda operatives in Syria.

But Comey said Wednesday the threat from the Khorasan Group has been ‘significantly diminished’ by military strikes.