LONDON -- A 32-year-old British man is being treated as the sole suspect in a stabbing attack on a train in eastern England Saturday evening, police said on Sunday.

Earlier, police said they had arrested a 32-year-old man and a 35-year-old man, both British nationals, but the latter has now been released without further action.

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"The 32-year-old man was arrested on suspicion of attempted murder and remains in police custody," police said, adding that a knife was recovered at the scene.

At least 11 people were stabbed on board the train travelling from Doncaster to London before it made an unscheduled stop at Huntingdon station to allow police to intervene.

An eyewitness told local media that they saw a man with a bloodied arm fleeing down a carriage, yelling "They've got a knife," with blood covering the chairs. Another witness said people were trampling over one another, and some were hiding in the toilets to escape.

A rail staff member who tried to stop the attack remains in life-threatening condition, while five others injured have now been discharged, according to authorities.

The injured employee's actions "were nothing short of heroic and undoubtedly saved many people's lives," British Transport Police said.

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Police added that there is currently nothing to suggest the attack was a terrorist incident. Counter-terrorism officers are assisting the investigation as authorities "work to establish the full circumstances and motivation."

British Prime Minister Keir Starmer described the "appalling incident" as "deeply concerning," urging people to follow advice from local authorities.

Defence Secretary John Healey told Sky News on Sunday that early assessments indicate the stabbing was an "isolated attack."