
Police told the BBC Ali, a British national believed to be of Somali descent, was being held under the UK's Terrorism Act, after he was initially arrested at the scene on murder charges. Under British law, that allows them to keep him in custody until Friday before official charges must be filed.
Amess, 69, was killed Friday while holding a regular weekly meeting with constituents at Belfairs Methodist Church in Leigh-on-Sea, Essex, about 40 miles east of London. The Conservative MP, first elected in 1983, was stabbed multiple times.

According to the BBC, Ali was referred a few years ago to the country's counter-terrorism program, called Prevent, which works to stop people being radicalized. It's not clear how long he spent in the program, and he was never a formal "subject of interest" to the Security Service, MI5, the outlet reported.
The Telegraph reported that Amess, a father of five, had received an "upsetting" threat in the days before the fatal attack. The threat was reported to police, but the news outlet said cops were not linking the threat to his death.
Comment: As just one example, during the lockdowns the police were deployed to stake out a hairdressers suspected of breaking lockdown rules, however when an MP receives a threat, which they consider serious enough to report, they didn't consider it worthwhile enough to spare a few officers?
Ali is believed to have acted alone, The Telegraph reported.
A candlelit vigil was held in Leigh-on Sea on Saturday night to mark the life of Amess. Earlier, Prime Minister Boris Johnson was one of many officials who paid respects to Amess at the church where he was killed.
Comment: This is just the latest incident where a potential terrorist is known to the security services, sometimes under constant surveillance, yet, somehow, they manage to 'slip' under the radar and go on to commit heinous crimes. Below are just a few examples of how this is a repeating problem: