Eleven people were treated in hospital after a mass stabbing on a moving high-speed train from Doncaster to London on Saturday evening.

Police on Sunday evening that five of these people had been discharged, with one still in a life-threatening condition.

Two men were initially arrested but, by Sunday evening, one of them had been released without charge. The other – a 32-year-old black British national, born in the UK – remains in police custody on suspicion of attempted murder, British Transport Police (BTP) said.

Shortly after the attacks began, the train made an unscheduled emergency stop at Huntingdon station in Cambridgeshire, where, according to witness accounts, police officers Tasered a man carrying a knife.

A member of LNER rail staff has been described as a hero after he tried to stop the attacker. He remains in hospital with life-threatening injuries.

Police said the staff member “undoubtedly saved many people’s lives”.

Deputy chief constable Stuart Cundy, of BTP, said on Sunday evening: “This was a horrific attack that has had a wide impact. My thoughts and those of everyone in British Transport Police are with those injured and their families – especially the brave member of rail staff whose family are being supported by specialist officers.

“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.”

Earlier on Sunday Supt John Loveless said: “At this stage, there is nothing to suggest that this is a terrorist incident.”

The defence secretary, John Healey, described it as an “isolated attack”, telling Sky News: “The early assessment is that this was an isolated incident, an isolated attack.

“So there’s no reason for the rest of us not to get on with our lives, get on and travel to the places we need to get to.”

Where and when did it happen?

The attack took place on the 6.25pm LNER train from Doncaster to London King’s Cross.

British Transport Police say they were called to reports of multiple stabbings at 7.42pm on Saturday evening after passengers pulled the emergency alarms. Passengers have said the attack started about 10 minutes after the train left Peterborough station.

The train driver made an unscheduled stop at Huntingdon station, where police and paramedics were gathered.

What do me know about the stabbings?

Witnesses have described “pure panic” after a man with a large knife began stabbing people.

Olly Foster, who was one carriage away from the incident, told the BBC that he heard people shouting “run, there’s a guy stabbing literally everyone and everything”. He first thought it might have been a Halloween prank.

A number of accounts have described there being “blood everywhere” and people trying to get away. Some hid in the train’s toilets.

What have police said?

British Transport Police say they were called to reports of multiple stabbings at 7.42pm and attended Huntingdon station immediately, along with paramedics.

Armed police from the Cambridgeshire force, which has its headquarters nearby, boarded the train and arrested two people who remain in custody.

Supt John Loveless said it was “a shocking incident” to which police and paramedics had responded immediately.

He said counter-terrorism police had initially supported the operation, though the incident was no longer being treated as terrorism-related. Rail passengers, he said, would see a heightened police presence across the network on Sunday.

The force said it had at one point declared “Plato” – the national code word used by emergency services when responding to a “marauding terror attack” – but later rescinded it.

What has the reaction been?

King Charles said in a statement: “My wife and I were truly appalled and shocked to hear of the dreadful knife attack that took place on board a train in Cambridgeshire last night. Our deepest sympathy and thoughts are with all those affected, and their loved ones.”

The prime minister, Keir Starmer, described the incident as “appalling” and “deeply concerning”. He added: “My thoughts are with all those affected, and my thanks go to the emergency services for their response.”

The home secretary, Shabana Mahmood, said: “After last night’s horrific attack, my thoughts today are with the victims, their friends and family.

“We now know this attack is not being treated as terrorism, and that two British-born, British nationals have been arrested.”

The Conservative leader, Kemi Badenoch, said: “This is an absolutely horrific attack and my thoughts are with all those affected including the emergency responders at the scene.”

The MP for Huntingdon, Ben Obese-Jecty, called the incident “appalling and devastating”.

How have train services been affected?

LNER, which operates east coast mainline services in the UK, said disruption to its services between London King’s Cross and Lincoln, Doncaster, Leeds, Bradford Forster Square and Harrogate, was expected to last until Monday, with passengers advised to defer their travel where possible.

Unused weekend LNER tickets dated either 1 November or 2 November will be valid until Tuesday 4 November.