London ? The police chief warned deadly terror cells could strike any time as thousands of officers flooded the transit system, made more arrests and grilled suspects Thursday in their biggest investigation ever. Exhausted police faced their greatest challenge since World War II.

With sleeper cells still thought to be active, it’s “a race against time,” said Metropolitan Police Commissioner Sir Ian Blair.

Police nabbed a terror suspect linked to the deadly July 7 bombings after he crossed into Zambia from Zimbabwe, a Zambian official said. Authorities also arrested nine new suspects in Britain and kept up interrogations of a captured bomber from the July 21 attacks.

“It does remain possible that those at large will strike again,” Blair said. “There are many thousands of police officers trying to ensure the safety of Londoners.”

Blair would not say how many of the force’s 30,000 officers were on duty Thursday, exactly three weeks since the July 7 bombings – which killed 56 people including four bombers – and one week since the botched July 21 bombings.

But he called it “the largest investigation the Met (Metropolitan police) has ever mounted” and “the greatest operational challenge that the Metropolitan police service has faced since the Second World War.”

Separately, the British Transport Police scrambled their largest-ever deployment to patrol the rail network. Though spokesman Simon Lubin refused to reveal how many officers were deployed, he said 1,300 officers in the capital and about the same number across Britain were working longer hours, and some vacations were canceled.

Blair said his officers were exhausted, with some “very tired faces” around Scotland Yard. But they were determined as they hunt for three more men suspected of taking part in the July 21 attack and anyone that might have helped them. Police have 20 people in custody.