A New York congressman shocked his Long Island audience on Sept. 12 by saying that as many as 40 Americans have returned to the U.S. after fighting for the ISIS terror army overseas, and that the Federal Bureau of Investigation has them all under surveillance.

Democratic Rep. Tim Bishop made the remarks during a breakfast speech to Long Island Metro Business Action, a regional economic development organization.

'One of the concerns is the number of U.S. citizens who have left our country to go join up with ISIS,' he said. 'It is believed there have been some number up to 100 that have done that.'

'It is also believed that some 40 of those who left this country to join up with ISIS have now returned to our country. Those 40 are under FBI attention and surveillance. So they are known and they are being tracked by the FBI.'

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New York Democratic Rep. Tim Bishop told the Long Island Metro Business Action group on Sept. 12 that 40 Americans have returned to the U.S. after going overseas to fight for ISIS, and that the FBI has all of them under surveillance

The Pentagon says only about a dozen Americans have gone overseas to join up with ISIS, but Rep. Bishop said more than three times that number have already gone and returned to the US

American jihadis, part of a swelling 'foreign fighter' contingent aiding the self-designated Islamic State of Iraq and al-Sham (ISIS) could put their ideologies and experience to work in the United States, a fact that has some in Congress clamoring for easier ways to revoke their citizenship.

The Pentagon said two weeks ago that only about a dozen Americans had left the country to join up with ISIS, while as many as 100 are suspected of taking up arms with a wider range of Islamist extremist groups in the Middle East and Africa.

Although Bishop cited a larger number of confirmed U.S. citizens who have come back into the United States after fighting for ISIS, he insisted that they don't pose a serious threat.

'At the present time,' he said, 'the intelligence is ISIS does not present a threat to the homeland, although that is not something that will remain static going out into the future.'

'But there is a concern,' he allowed, 'that it could metastasize in such a way that it could become a threat to the homeland.'

Bishop backed President Barack Obama's approach to defeating the terrorist would-be country, which he said 'is a very, very complex and very, very dangerous threat.'

Referring to remarks Obama delivered on Sept, 10, he said, 'I think the plan that the president outlined the other night [in his national speech] is a good plan.'

The president 'is correct to recognize the threat, and it clearly is a threat to the stability of the Middle East, a region that is already remarkably unstable.'

The Washington Free Beacon first reported on Bishop's remarks.

Up to 12,000 so-called 'foreign fighters' from 50 countries have joined with various Muslim militant groups, including those who fight alongside ISIS in Iraq and Syria, according to the U.S. State Department.

ISIS itself can now muster 31,000 troops, the CIA reported on Sept. 17. That number has grown dramatically since June 29, when ISIS declared itself a 'caliphate,' or an Islamic nation led by a supreme religious authority.

On Thursday, Senate Democrats thwarted legislation that would have revoked the citizenship of Americans who swear allegiance to terror groups that threaten the U.S.

Foreign fighters: As many as 12,000 soldiers in the 31,000-man ISIS terror army have traveled into Syria or Iraq to join the Islamist group in battle

FBI Director James Comey said he would welcome a legal tool to keep ISIS fighters from returning to the US by stripping them of citizenship, but fears it would be hard to argue cases in court without publicly describing classified intelligence

Democratic Sen. Mazie Hirono of Hawaii put a halt to Texas Republican Sen. Ted Cruz's attempt to fast-track the bill, objecting that her party needed more time to consider its legal implications.

James Comey, the director of the FBI, told a Capitol Hearing Thursday that he would be open to seeing such a law passed. But he expressed concerns that it could be impractical to enforce in court since much of the evidence would likely be classified.

'How would I protect sources and methods?' he asked. 'How would we be able to use, if at all, classified information to make the showing that would be necessary [in court]?'

Rep. Ed Royce, a California Republican, told Secretary of State John Kerry in a separate hearing that Congress should do more to restrict the travel of ISIS fighters who hold American passports.