By GUY DAVIES, ABC News

(LONDON) — An American national was among the three victims of a terrorist stabbing in the U.K. on Saturday night, Ambassador Woody Johnson confirmed.

“I offer my deepest condolences to the families of those killed in the attack on June 20,” Ambassador Johnson posted on Twitter. “To our great sorrow, this includes an American citizen. Our thoughts are with all those affected. We condemn the attack absolutely and have offered our assistance to British law enforcement.”

Three people were killed and a further three were injured in the mass stabbing in Forbury Gardens, Reading, a town just outside of London, on Saturday. A 25-year-old man has been arrested in connection with the attack.

Neil Basu, the assistant commissioner of the Metropolitan Police, said that the stabbing had been declared a terrorist incident as a result of an overnight investigation.

“A man attacked members of the public with a knife,” he said at a press conference on Sunday morning. “Members of the public provided first aid. Counterterrorism officers also attended the scene immediately. I can confirm this was declared a terrorist incident as a result of the investigations through the night. A 25 yr old man was arrested last night on suspicion of murder.”

The suspect in custody has been named as Khairi Saadallah in the British media.

The man was believed to have acted alone, he added.

“I’m appalled and sickened that people should lose their lives in this way,” Prime Minister Boris Johnson said. “I’m appalled and sickened that people should lose their lives in this way. Our thoughts are with the families and friends of the victims. Thank you to Thames Valley Police for their bravery in tackling the suspect.”

Detective Chief Superintendent Kath Barnes, the head of counter terrorism policing in the south-east of England, said the investigation is moving at a “fast pace” on Sunday night.

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