One witness told reporters that several people were seated together on a grassy area when a man approached, shouted and attacked with a knife.
Bennett was killed along with his friend James Furlong, a history teacher at Holt School in nearby Wokingham. The third slain victim has not yet been identified.
Three other people were also seriously injured.
Police arrested a 25-year-old suspect within five minutes.
Although authorities have not released the identity of the accused, several major British news outlets named the suspect as Khairi Saadallah, a Libyan asylum-seeker living in Reading, according to The Associated Press.
Police have two weeks to question the suspect without charge because he was arrested under the Terrorism Act.
Police said they were not seeking anyone else in the attack.
Family: ‘We’re mourning’
The Philadelphia Inquirer quoted the father of Ritchie-Bennett as saying his son had moved to England from the U.S. about 15 years ago. His father, Robert Ritchie, said his son worked for a law firm in London before taking a job about 10 years ago at a Dutch pharmaceutical company that had its British headquarters in Reading. He called him an “absolutely fabulous guy” whom he loved with all his heart.
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“We’re mourning, and we’re trying to decide what we’re going to do,” he told the Inquirer. “It’s 3,500 miles away. They are still in lockdown over there with the coronavirus, and I don’t know what else to say.”
As news of the deaths seeped out into the community, friends grieved. Martin Cooper, chief executive of LGBT+ charity Reading Pride, said Furlong and Ritchie-Bennett were great supporters of the community.
“Their loss is a tragedy to so many people,” said Cooper, who often socialized with them. “They should be remembered as extremely friendly gentlemen who were always fun, engaging and a pleasure to be around. They were their own little support network for anybody to offload their troubles and concerns, and gave great advice.”
More about the suspect
Saadallah, meanwhile, had been depressed and received psychological treatment because of the chaos in Libya after the NATO-backed uprising that toppled and then killed dictator Moammar Gadhafi, a family member in Tripoli told The Associated Press.
The relative said Saadallah was born to a wealthy family in Tripoli. He lived in a villa and went to private schools in Libya. Though he supported Gadhafi's ouster, he became disillusioned with the chaotic aftermath.
The British Counter Terrorism Police is investigating after Thames Valley Police initially thought the attack to be random.
— Information provided by The Associated Press was used to supplement this report.