A 40-year-old man arrested on suspicion of terrorism after three people has been arrested in Manchester under the Mental Health Act.
The police spokesman said this evening.
He added: "The investigation will continue in order to fully understand the motives behind the attack.
"There is no doubt to be any greater threat to the communities of Greater Manchester and the attacker is believed to have acted alone."
It comes as three people were stabbed at the Arndale shopping center just after 11.15am.
A 19-year-old woman and another female were both taken to hospital with stab injuries. Their conditions have been described as stable by the force.
A man in his 50s was also taken to hospital with stab wounds, while a woman in her 40s was treated by paramedics at the scene but was not stabbed.
A spokesman GMP said: "All but one have been discharged.
"The man in his 50s is not believed to have life threatening injuries."
Witnesses earlier described a scene of terrorists as shoppers, some with injuries, sought refuge in stores amid the panic.
One shop worker, who gave his name only to Jordan, 23, said: "A man was running around with a knife lunging at multiple people, one of which came into my blind visibly shaken with a small graze.
"Soon after, security staff told all the staff to close their doors and move the public to the back of the stores."
Jordan, who has been working at the center for a few months, said the attack took place on the ground floor.
Freddie Houlder, 22, from Market Drayton was in the Arndale Center when he heard "a load of screams just outside" the shop he was in.
He said, "Come back to the shop and tell others."
Mr Houlder added: "Luckily she had quite a thick jacket – she thought it was a fake knife because it was so fast and easy it was a real knife and she was burst in to tears."
He said he was evacuated several minutes later than he saw "a bunch of curtains covering someone seriously injured or dead, I do not know".
"I definitely do not think it was gang violence because this guy was going around trying to stab random people, I believe."