British police said they have arrested 12 people in east London over the attack in the London Bridge area.
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Police said in a statement that the arrests were made Sunday morning in Barking by counterterrorism officers.
Seven people died in the Saturday night attack and 48 were wounded. A British Transport Police officer and an off-duty officer were among the injured. In addition, three attackers were shot dead by police.
Britain's interior minister said the country's terrorism threat level will not be raised because police believe there are no perpetrators still on the loose. Home Secretary Amber Rudd said that "we don't believe there are additional elements" at large.
The level was raised to "critical" for several days after the March 22 concert bombing in Manchester, as police raced to track down the attacker's possible accomplices.
It now stands at "severe," meaning an attack is highly likely.