Journalist Piers Morgan grilled London Mayor Sadiq Khan over his police department's "priorities" in the wake of the city's latest terrorist attack.

Khan said on "Good Morning Britain" that "just over half" of the 400 Britons who trained with terror groups in the Middle East have returned to the U.K. capital.

"How are we letting them back in the U.K.?" co-host Susanna Reid asked. "[They] actually fought... potentially against our troops."

"Where are they?" Morgan asked.

London Mayor Sadiq Khan: UK Shouldn't 'Roll Out the Red Carpet' for Trump

Bolling Blasts Senator: 'Dumbest Comment I Heard in a Long Time'

'I'm Asking the Questions': Sparks Fly as Dem Sen. Scolds Gen. Kelly at Hearing

Morgan again asked why Khan cannot tell the police to follow every person "who has come back from a warzone."

Khan said his police cannot follow 400 people, eliciting another stern question from Morgan.

Khan said his police department must "prioritize" because the British government has cut its resources.

"What could be a bigger priority than people coming back from a Syrian battlefield with intent to harm British citizens?" Morgan asked.

Watch the full exchange above.

Should British Police Officers Be Armed With Guns?

Former CIA Officer on the Challenges of Terror Investigations

'The Left Is Coming Unglued': Watters Slams CNN Host For Obscene Trump Tweet