The Department of Justice has “hundreds” of ongoing terrorism-related investigations into people who came to the U.S. as refugees, according to the nation’s top cop.

Attorney General Jeff Sessions made the comment early Thursday during remarks from New York City about national security, just days after a man inspired by the Islamic State carried out a deadly attack near the World Trade Center with a rented truck.

Sessions argued the manpower and resources that go into preventing terror attacks, both at home and abroad, are strained by the inability of the Trump administration to fully carry out the president’s executive order travel ban.

“The president is determined to keep terrorists and their sympathizers from infiltrating our country,” Sessions explained, adding, “And he knows that, since 9/11, most of those convicted in our courts for international terrorism-related crimes have been foreign-born.”

The executive order was part of Trump’s “legal right,” Sessions said.

“The countries covered by this order have failed, or are unable, to provide us the information necessary for proper vetting of their nationals. How do you vet people from North Korea? How do you vet people from Syria, where war and violence continues?” Sessions argued.

Following Tuesday’s terror attack, Trump vowed more “extreme vetting” of immigrants and refugees coming into the country, especially after learning the alleged assailant, Sayfullo Saipov, came into the U.S. through the diversity visa program.

Those tighter restrictions, Sessions said, are because the president “understands that we continue to face grave security threats from a number of groups and he is not afraid to talk openly and directly about it.”

Trump’s “separate action to reduce the flow of refugees into this country will reduce the likelihood of potentially dangerous people getting here,” Sessions continued. “It will also take some of the pressure off of the FBI and our local law enforcement by control and vetting more carefully those requesting entry before they are admitted.”