PA MediaPassengers travelling from Doncaster to London were attacked in a mass stabbing on a train on Saturday night.
Eleven people were injured and received hospital treatment. One - a member of the train staff - is in life-threatening condition.
Two British men in their 30s were initially arrested while police said there was"nothing to suggest" it was a terror incident.
One has since been released with no further action.
Witnesses reported that police used a Taser on one man who was holding a knife.
Prime Minister Sir Keir Starmer said the attack was"appalling" and "deeply concerning".
Here is what we know so far about what happened.
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Where did the stabbings happen?
The attack took place on the 18:25 GMT London North Eastern Railway (LNER) service from Doncaster, South Yorkshire to London's King's Cross station.
Passengers said at least one person brandishing a knife began stabbing people on the train after it passed through Peterborough in Cambridgeshire.
Cambridgeshire Police received the first call from passengers on board at 19:39, and the British Transport Police (BTP) was also alerted at 19:42.
The train made an unscheduled stop at Huntingdon, which is some 15 minutes from Peterborough by train.
Altogether, the incident lasted roughly 10 to 15 minutes.
Armed police boarded the train and arrested the two men within eight minutes of the call to BTP.
A large emergency service response was also sent to the scene, including air ambulances.
Police said on Sunday that a knife was recovered from the scene.
The driver of the train, who has since been identified as Andrew Johnson - a former Chief Petty Officer in the Royal Navy - contacted the control room from his cab to get the train diverted from the fast track to the slow track when the alarm was raised.
It meant the train could stop at a platform in Huntingdon.
Uninjured passengers were interviewed by police and some boarded a coach bound for London.
Huntingdon MP Ben Obese-Jecty told the BBC there were about 10 ambulances, several fire engines and"well over 20 police cars" when he first arrived at the scene just after 21:00.
National Rail said the station would remain closed until the end of the day on Monday.
The empty train was seen at the platform on Sunday along with a police presence and forensics tents.
What do we know about the suspects?
Following the incident, Supt John Loveless of the British Transport Police said two UK nationals, men aged 32 and 35, were arrested at the scene.
In a later update on Sunday evening, British Transport Police said the 35-year-old, from London, had been released with no further action.
The force said it had been been reported"in good faith" to responding officers that he had been involved.
"Following inquiries we can confirm that he was not involved," it added.
The 32-year-old remains in police custody after having been arrested on suspicion of attempted murder.
He has not been named but police previously said he is a black British national.
The suspect is from Peterborough and boarded the train at Peterborough station.
Deputy Chief Constable Stuart Cundy said specialist detectives are looking into the background of the suspect and the events that led up to the attack.
What do we know about the victims?
Emergency crews took 10 people to Addenbrooke's Hospital in Cambridge - which is some 30 minutes away from the train station - and one person later went to the hospital for treatment.
Shortly before 18:00 on Sunday, police said five people had been discharged from hospital and one remained in a"life-threatening condition".
"This casualty is a member of LNER rail staff who was on the train at the time and tried to stop the attacker," the police said.
"Detectives have reviewed the CCTV from the train and it is clear his actions were nothing short of heroic and undoubtedly saved many people's lives."
No victims have been named.
What have eyewitnesses said?
Witnesses told the BBC of panic and confusion as passengers ran through the carriages, some wounded and bloodied.
Alistair Day, 58, told the BBC he hid in the buffet car with about 11 other passengers while the attacker attempted to gain entry.
He ran to the car after seeing"a guy flailing out - a fracas with arms going everywhere".
He said one passenger turned to him and calmly said he had been stabbed in the chest:"He had blood all over him, so we put pressure and stuff and held him."
Olly Foster said he heard people shouting "run, there's a guy stabbing literally everyone and everything" and thought at first it might have been a Halloween prank.
He saw an older man with gashes on his head and neck after he"blocked" the attacker from stabbing a younger girl. Passengers then used their jackets to try and staunch the bleeding.
Nottingham University student Amira Ostalski was on the train with her friend when she saw a man leap out of his seat a few rows ahead of her.
"Obviously he was stabbed, but at that time I didn't know," she said.
"Everyone started shouting - he's got a knife, there's blood."
She ran with her friend to the back of the train as the attacker went in the opposite direction.
"I got a metal tray [...] I wanted to protect myself," she told the BBC.
"Police came and detained him right in front of my eyes," she said.
Ms Ostalski added she would feel"vulnerable" if she were to sit on a train again.
3:27
Another witness, Wren Chambers, said one person had been stabbed in the arm and bolted down the train to alert others while another shouted"someone's got a knife".
Some passengers hid inside the toilets while others swarmed towards the front of the train.
London Underground worker Dean McFarlane said he saw multiple people running down the platform bleeding, with one man in a white shirt"completely covered in blood".
Another passenger named Steve told the BBC an alarm sounded just after he boarded the train in Peterborough with his children.
"Then a woman came over the Tannoy and said, 'we're aware there's been an incident, just keep yourselves safe,' which is scary to hear," he said.
Steve said the attack happened at the other end of the train. He was in carriage B, and was told the incident took place in carriage J.
Once the train stopped, he hammered on the doors and"sprinted" away as passengers "piled out" of the train.
He said it was"unnerving" not knowing what was happening.
1:39
What have police said?
British Transport Police (BTP) declared a major incident and initially said counter-terrorism officers were supporting the investigation"to establish the full circumstances and motivation for this incident".
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.
Supt Loveless later said at a press conference that there was"nothing to suggest this is a terrorist incident".
Defence Secretary John Healey told the BBC early reports suggested it was an isolated incident.
He added that this was a service he used often and he had travelled along the same route just hours before the attack.
What has the reaction been?
Prime Minister Sir Keir Starmer said the"appalling incident on a train near Huntingdon is deeply concerning".
He wrote on X:"My thoughts are with all those affected, and my thanks go to the emergency services for their response."
Home Secretary Shabana Mahmood said her thoughts were with the victims, their friends and family, and praised emergency workers who"responded rapidly, with the utmost professionalism and saving lives".
King Charles issued a statement saying he and Queen Camilla extended their"deepest sympathy" to those affected and their loved ones.
"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," his statement read.
"We are particularly grateful to the emergency services for their response to this awful incident."
Conservative leader Kemi Badenoch told the BBC she was "horrified" by the attack, saying:"I can only imagine how frightening it must have been to be in an enclosed environment with someone rampaging in that fashion."
She praised the emergency response to the incident and urged people not to speculate, while questioning why"we're seeing more and more violence on our streets" despite efforts to tackle knife crime.
Meanwhile, the Liberal Democrat leader Ed Davey praised the"extraordinary bravery" of passengers who stepped forward to protect others, while Reform UK's head of policy Zia Yusuf told the BBC said people were being"traumatised" in this country due to "appalling knife crime".
What is happening with train services?
LNER urged passengers to check before travelling on Sunday due to possible cancellations or changes to services.
Tickets for Saturday and Sunday will be valid until Friday, and those who no longer want to travel this week can get a refund.
Lines have reopened between Hitchin and Peterborough. Great Northern and Thameslink trains between London Kings Cross and Peterborough will run, but there may be delays or cancellations.
A rail replacement service is in place to and from Huntingdon.
Transport Secretary Heidi Alexander said passengers would see a"high visibility presence" of police at stations and on trains throughout the day "to reassure the public".
Government sources told the PA news agency there will be a"surge" in police presence until at least Tuesday in core transport hubs across the country such as London, Birmingham and Manchester, in addition to Huntingdon.
