LONDON (AP) — Counterterrorism investigators searched two homes Monday and detained “a number” of people in the investigation into a van and knife attack in the heart of London that left seven people dead.

Dozens were injured, 21 of them critically, in the attack. The three attackers, who wore fake suicide vests, were shot to death by police. The Islamic State group has claimed responsibility.

London’s police chief has said the attackers have been identified, but the names have not been released.

Metropolitan Police Commissioner Cressida Dick told Sky News on Monday she would not release further details in what she described as a fast-moving investigation, not would she say whether authorities were familiar with the men ahead of the attack.

Prime Minister Theresa May warned that the country faced a new threat from copycat attacks.

The country’s major political parties temporarily suspended campaigning with only days to go before the general election. May said the vote would take place as scheduled Thursday because “violence can never be allowed to disrupt the democratic process.”