Updated

The stabbing rampage in a British park that left three people dead is now being treated as a terrorist attack, police said.

Dean Haydon, the U.K.’s coordinator for counterterrorism policing, said Sunday morning that counterterror detectives were taking over the investigation into the attack in the town of Reading, west of London. Police had earlier said the motive was unclear.

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Three people were killed and three others seriously injured when a 25-year-old man, who is in custody, went on a rampage in Reading’s Forbury Gardens Park on Saturday evening.

The man is believed to be Libyan, sources told the BBC and Sky News. He reportedly served time in prison in England for a minor offense.

Police stand guard at the Abbey gateway of Forbury Gardens park in Reading town center Sunday following Saturday's stabbing attack in the gardens. (Jonathan Brady/PA via AP)

Police stand guard at the Abbey gateway of Forbury Gardens park in Reading town center Sunday following Saturday's stabbing attack in the gardens. (Jonathan Brady/PA via AP)

Lawrence Wort, a personal trainer who witnessed the attack, said the park was crowded when suddenly a lone person "shouted some unintelligible words and went around a large group of around 10, trying to stab them.”

“He stabbed three of them severely in the neck and under the arms, and then turned and started running toward me, and we turned and started running,” Wort said.

A heavy police presence remained at the scene Sunday as officers patrolled the cordoned off roads leading to the park. Blue-and-white tents were erected near the site of the attack.

Armed police officers work at a block of flats off Basingstoke Road in Reading after an incident at Forbury Gardens park in the town center of Reading, England, early Sunday. (Steve Parsons/PA via AP)

Armed police officers work at a block of flats off Basingstoke Road in Reading after an incident at Forbury Gardens park in the town center of Reading, England, early Sunday. (Steve Parsons/PA via AP)

Overnight, heavily armed officers raided an apartment about a mile away from the park, where the suspect was believed to be living, Sky News reported. After all the residents were safely evacuated, a loud bang was heard from inside the building.

The incident came hours after a Black Lives Matter demonstration at Forbury Gardens, but police said there was no connection between the attack and the protest.

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Prime Minister Boris Johnson was meeting with security officials, police, and senior ministers Sunday morning for a briefing on the investigation, the prime minister's office said.

Johnson on Saturday said his “thoughts are with all of those affected by the appalling incident in Reading."

Marie Castro, who lives in nearby Slough and works at a coffee shop in Reading, told the BBC that the attack "doesn't seem right for Reading.”

Police patrol the Abbey gateway of Forbury Gardens park in Reading town centre on Sunday following Saturday's stabbing attack in the gardens. (Jonathan Brady/PA via AP)

Police patrol the Abbey gateway of Forbury Gardens park in Reading town centre on Sunday following Saturday's stabbing attack in the gardens. (Jonathan Brady/PA via AP)

"It's multicultural and really friendly,” she said. “I was really shocked when I heard the news".

Britain has been hit by several terror attacks in recent years, including a suicide bombing at an Ariana Grande concert in Manchester in 2017 that killed 22 people and two deadly vehicle and knife attacks in London the same year.

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Britain’s official terrorism threat level stands at “substantial,” the middle level on a five-rung scale, meaning an attack is likely.

The Associated Press contributed to this report.