Rep. Tim Bishop (D., N.Y.) warned during a recent speech that up to 40 radicalized U.S. citizens who have fought alongside the Islamic State of Iraq and Levant (ISIL or ISIS) have already returned to the United States, where they could pose a terrorist threat.

Bishop claims that of the 100 or so Americans who have traveled to the Middle East to join ISIL's ranks, some 40 have returned and are currently being surveilled by the FBI, according to his remarks, which were filmed and uploaded to YouTube last week.

"One of the concerns is the number of U.S. citizens who have left our country to go join up with ISIS," Bishop said during the speech. "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," Bishop said, eliciting shocked responses from some in the crowd.

These 40 individuals, Bishop said, "are under FBI attention and surveillance. So they are known and being tracked by the FBI."

Lawmakers have warned that radicalized ISIL fighters could clandestinely enter the United States through the porous southern border.

U.S. officials with the Department of Homeland Security (DHS) and other security agencies have said that while the southern border poses a risk, they are more concerned about lone wolf attacks in which an isolated radicalized individual with sympathy to ISIL carries out a solo terror attack.

Bishop said that while ISIL poses and extreme threat, it is not yet capable of attacking the United States, though that could change as the group grows in strength and resources.

"It is a very, very complex and very, very dangerous threat and I think the plan that the president outlined the other night [in his national speech] is a good plan," Bishop maintained.

President Obama "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," Bishop said.

"It is not yet a threat to the homeland, but there is a concern that it could metastasize in such a way that it could become a threat to the homeland," Bishop said, echoing concerns expressed by lawmakers on both sides of the isle.

"But at the present time, 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," Bishop said, describing the regional upheaval as "more of a political conflict in the Arab world then a sectarian conflict" between Muslims.

DHS has said that while there is no evidence of a direct threat by ISIL to the United States, it is aware that the group's affiliates have been discussing the possibility of crossing the southern border.

"There have been Twitter, social media exchanges among ISIL adherents across the globe speaking about that as a possibility," Francis Taylor, under secretary for intelligence and analysis at DHS, told senators during a recent hearing.

Other U.S. officials also have discussed the possibility of an ISIL adherent carrying out an attack on the United States.

"We remain mindful of the possibility that an ISIL-sympathizer—perhaps motivated by online propaganda—could conduct a limited, self-directed attack here at home with no warning," Matthew Olsen, director of the U.S. National Counterterrorism Center, said in a speech earlier this month.

"We have seen ISIL use a range of media to tout its military capabilities, executions of captured soldiers, and consecutive battlefield victories," Olsen said. "More recently, the group’s supporters have sustained this momentum on social media by encouraging attacks in the U.S. and against U.S. interests in retaliation for our airstrikes. ISIL has used this propaganda campaign to draw foreign fighters to the group, including many from Western countries."