U.S. Department of Homeland (DHS) Security Secretary Jeh Johnson, while testifying before a House panel today, indicated that the government’s ability to detect terrorists inside the homeland is hindered by sanctuary cities that shelter illegals.

Johnson added that America must remain vigilant of the extremist group Islamic State, also known as ISIS and ISIL, or other terrorists potential infiltrating the U.S. through its southern border, adding that his department could use more resources to prevent that from becoming a reality.

The DHS chief made those comments today during a House Homeland Security Committee hearing on worldwide threats to the U.S..

“We have a pretty good ability through law enforcement intelligence, homeland security means to identify individuals, including undocumented [ones], who are people of suspicion, suspected terrorists, and the FBI proves that time and [time] again,” Johnson told members of the panel. “I do think that in any situation where there are large number of people who are undocumented, there is a risk that it hinders our ability to track those individuals, which is why from my homeland security perspective, I want to see those people come forward and get on the books so that I know who they are.” “So, if what you’re suggesting is that the risk to homeland security grows when there are larger numbers of undocumented people in any one place, in any crowded area,” he continued. “I can’t disagree with that.”

The DHS chief’s comments came in response to a query from Rep. Paul Braun (R-GA).

Rep. Braun noted that various cities and the state of California have declared themselves sanctuaries for illegals inside the U.S. He then asked the DHS secretary if he considered that a risk to U.S. homeland security.

In response to a question by the committee’s ranking-member, Rep. Bennie Thompson (D-MS), regarding the threat of ISIL crossing into the U.S. through the southwest border, Johnson said there is “no specific intelligence or evidence” to suggest that.

However, he added, “Having said that, we need to be vigilant. We do need to be aware of the risk of potential infiltration by ISIL or any other terrorist group and we have tools in place to monitor that and to do that.”

Matthew Olsen, the director of the National Counterterrorism Center who testified along Johnson, added, “There has been a very small number of sympathizers with ISIL who have posted messages on social media about this, but we see nothing to indicate that there is any sort of operational effort or plot to infiltrate or move operatives from ISIL into the United States through the southern border.”

Rep. Sheila Jackson-Lee (D-TX) later asked the DHS secretary if he had enough resources to prevent the Islamic State from infiltrating America through the U.S.-Mexico border.

Although security resources on the U.S. borders have reached “unprecedented” proportions, “we can use more,” responded Johnson.