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Ex-DHS official on UK terror arrests: Lone wolf attackers can still have support

London attacker may not have acted as 'lone wolf': Expert

British police are searching for clues about whether the suspected terrorist who carried out the deadly London attack near Parliament had help, a former Department of Homeland Security official said Thursday.

Law enforcement has searched six addresses and made eight arrests in raids as part of an investigation into Wednesday's attack, which killed three people dead and injured 40, including one American, London's Metropolitan Police has announced. The attacker also was killed.

"What British authorities are trying to do right now is ascertain whether he had any support mechanism," Jack Thomas Tomarchio, a former deputy under secretary for intelligence and analysis operations, said on CNBC's "Squawk Box." "That's why you see these ... other people arrested."

Tomarchio, a senior fellow in FPRI's Center for the Study of Terrorism, said British police will try to use some forensics: Look at the attacker's cellphone and laptop if they have it, just to harvest information.

"Often you find that in cases like this, there is often a support mechanism, and a terrorist doesn't usually act totally as a lone wolf," he said.

Metropolitan Police counterterrorism chief Mark Rowley said the attacker, who plowed a car into pedestrians on London's Westminster Bridge and then fatally stabbed a police officer before being shot, acted alone and was "inspired by international terrorism."

Rowley revised the death toll from five to four — the attacker, the police officer and two civilians. He said seven of 29 people who required hospitalization were in critical condition.

Prime Minister Theresa May said the man responsible for Wednesday's attack was a "peripheral figure" known to the MI5 British spy agency.

May added that there was no prior evidence of the plot or the perpetrator's intent.

—CNBC's Karen Gilchrist and The Associated Press contributed to this report.