(CNN) Police investigating the stabbings of three people at a railway station in Manchester, in northern England, on New Year's Eve have arrested a 25-year-old man.

The suspect, who is being detained under the Mental Health Act, is now in a secure medical facility, Greater Manchester Police said. Police are confident the man acted alone in "the final stages of the attack," they said.

But police didn't rule out the involvement of others at an earlier stage. "The investigation will continue and will examine if anybody may have encouraged or assisted the man to commit the attack," they said in a statement posted Wednesday on Twitter.

A male police officer was stabbed in the shoulder, and another man and a woman were taken to a hospital with serious knife injuries after the "horrific" attack just before 9 p.m. (4 p.m. ET) Monday at the Manchester Victoria railway station, police said in an earlier statement.

"We experienced a horrific attack on people out to simply enjoy the New Year's Eve celebrations in Manchester," Manchester Chief Constable Ian Hopkins said in a statement.

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