British police said Sunday that one of two men arrested in connection with a mass stabbing attack that spread fear and panic on a London-bound train a day earlier has been released without charge, and that one person, a "heroic" member of the railway staff, remains in a life-threatening condition.

In a statement Sunday evening, police said the only remaining suspect is a 32-year-old British man who remains in custody on suspicion of attempted murder. A second man initially arrested as a suspect was released without charge after it was determined the 35-year-old was not involved.

Police said they are not treating the stabbings as an act of terror and are confident they are not looking for anyone else in connection with the attack. They have not disclosed a possible motive or the type of knife used.

"Our investigation is moving at pace and we are confident we are not looking for anyone else in connection to the incident," said Deputy Chief Constable Stuart Cundy of the British Transport Police.

"As would be expected, specialist detectives are looking into the background of the suspect we have in custody and the events that led up to the attack," he added.

Six people remained in hospitals Sunday, one of them in a life-threatening condition. Police said he is a staff member of the train`s operator, London North Eastern Railway, or LNER, which operates the East Coast Mainline services in the U.K.

"Having viewed the CCTV from the train, the actions of the member of rail staff were nothing short of heroic and undoubtedly saved people`s lives," said Cundy.

The five others injured during the attack on Saturday evening have been discharged from hospitals.

The two men were arrested eight minutes after the first emergency calls were made at 7:42 p.m. Saturday from aboard the train, where passengers had reported scenes of panic and chaos, with many running through the carriages and some seeking safety in the toilets.

Police said the suspect is from Peterborough and that he boarded the train at the town`s station, just a few minutes before it was forced into an emergency stop in Huntingdon, a market town around 120 kilometers (75 miles) north of London.

The attack took place as the 6:25 p.m. train from Doncaster in northern England to London`s King`s Cross station was about halfway through its two-hour journey, having just departed Peterborough.

Bloodied and confused passengers spilled out of the train at Huntingdon as dozens of police waited, some of them armed. A knife was recovered by officers at the scene.

During the immediate response to the attack, police said that "Plato," the national code word used by police and emergency services when responding to what could be a "marauding terror attack," was initiated. That declaration was later rescinded.

Passenger Olly Foster told the BBC he heard people shouting "run, run, there`s a guy literally stabbing everyone," and initially thought it might have been a Halloween prank - Saturday was the day after Halloween. But as passengers pushed past him to get away, he noticed his hand was covered in blood from a chair he had leaned on.

Following reports that some of those on board the train put themselves in harm`s way to protect others, Home Secretary Shabana Mahmood praised the "exceptional bravery of staff and passengers on the train."

King Charles III said he and his wife, Queen Camilla, sent their sympathies and thoughts to those affected and that they were "truly appalled and shocked to hear of the dreadful knife attack."

Passengers across the U.K.`s rail network saw a heightened police presence on Sunday, both on trains and at stations.