Manchester – A British probe into a stabbing at a Manchester railway station on New Year's Eve which injured three people is being treated as "a terrorist investigation", the city's police force said on Tuesday.The 25-year-old man who was arrested at the scene on Monday night is now being detained under a mental health law, Greater Manchester Police (GMP) said late on Tuesday.The suspect reportedly shouted "Allah" and "long live the Caliphate" during what GMP Chief Constable Ian Hopkins called a "horrific attack" on three people, including a police officer."(The) man arrested last night... following the incident at Manchester Victoria Station has been assessed by specialist medical staff and detained under the Mental Health Act," GMP said in a statement."The Counter Terrorism investigation remains ongoing."There is nothing to suggest the involvement of other people in this attack, but confirming this remains a main priority for the investigation," the force added.Officers are still searching an address in the northwestern English city "which is believed to be where the man had most recently been living", according to Hopkins.A man and a woman, both in their 50s, were being treated in hospital on Tuesday after sustaining serious but not life-threatening injuries to their abdomens, while the British Transport Police officer was stabbed in the shoulder, the GMP chief added.Prime Minister Theresa May thanked emergency services for their "courageous response" in swiftly tackling the suspect.She wrote on Twitter: "My thoughts are with those who were injured in the suspected terrorist attack in Manchester last night."'Pure fear'No group has claimed responsibility for the attack, which happened at around 20:50 when many revellers in the city would have been enjoying New Year's Eve celebrations.Witness Sam Clack, 38, a BBC radio producer, said the suspect shouted "Allah" before and during the attack.He also quoted the perpetrator as saying: "As long as you keep bombing other countries, this sort of shit is going to keep happening."The witness added he heard the "most blood-curdling scream" and looked down the platform to see the attack unfolding."He came towards me," he added. "I looked down and saw he had a kitchen knife with a black handle with a good 12-inch blade."It was just fear, pure fear."Clack said police used a stun gun and pepper spray before "six or seven" officers jumped on the man.Video footage of the incident shows him being overpowered by the officers.The police said he was arrested on suspicion of attempted murder and officers had recovered two knives from the scene.'Dreadful'The city's New Year celebrations went ahead in Albert Square despite the incident, with a fireworks display taking place as planned, though increased security was brought in.British Transport Police said on Tuesday its officers would be "highly visible through the national rail network".Manchester Mayor Andy Burnham paid tribute to the emergency services' response.He added the "vile attack" had "all the hallmarks... of an isolated incident".Manchester police assistant chief constable Russ Jackson said they were still trying to establish whether the suspect was a British national, and how he came to be at the station."We are obviously considering his mental health given how frenzied the attack was, its random nature," he added."There is wide reporting... about what the attacker allegedly said during the incident."However it's really important to stress we are retaining an open mind in relation to the motivation for this attack."Police said investigators were considering its closeness to the Manchester Arena, where a suicide bomber struck in May 2017, as potentially significant.The attack at a concert by US singer Ariana Grande killed 22 people and wounded 139.The perpetrator, Salman Abedi, 22, was born and raised in Manchester."That the incident happened so close to the scene of the terrorist attack on May 22, 2017 makes it even more dreadful," chief constable Hopkins said.