Passengers travelling from Doncaster to London were attacked in a mass stabbing on a train on Saturday night.
Ten people were injured and taken to hospital, nine of whom are believed to have life-threatening injuries.
Witnesses reported that police used a Taser on one man who was holding a knife. Two people were arrested and counter-terror police have joined the investigation.
Prime Minister Sir Keir Starmer said the attack was “appalling” and “deeply concerning”.
Here is what we know so far about what happened.
The attack took place on the 18:25 GMT London North Eastern Railway (LNER) service from Doncaster, South Yorkshire to London’s King Cross station.
Passengers said at least one person brandishing a knife began stabbing people on the train after it passed through Peterborough in Cambridgeshire.
Police received calls from passengers on board at around 19:40, alerting them to the attack.
The train made an unscheduled stop at Huntingdon – which is some 15 minutes from Peterborough by train.
Armed police then boarded the train and arrested two people, whose identities are not yet known.
A large emergency service response was also sent to the scene, including air ambulances.
Altogether, the incident was estimated to last roughly 10 to 15 minutes.
The 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.
The station remained shut on Sunday morning, as well as the A1307 directly outside.
The empty train was still at the platform, while a police presence and forensics tents could also be seen.
Emergency crews took 10 people to hospital, nine of whom are believed to have suffered life-threatening injuries, according to police.
Their identities are not yet known.
They were taken to to Addenbrookes Hospital in Cambridge, which is about a 30-minute drive from the train station.
Witnesses told the BBC of panic and confusion as passengers ran through the carriages, some wounded and bloodied.
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 to staunch the bleeding.
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 at Huntingdon bleeding, with one man in a white shirt “completely covered in blood”.
British Transport Police (BTP) declared a major incident and said counter-terrorism officers were supporting the investigation “to establish the full circumstances and motivation for this incident”.
Ch Supt Chris Casey said: “We’re conducting urgent enquiries to establish what has happened, and it could take some time before we are in a position to confirm anything further.
“At this early stage it would not be appropriate to speculate on the causes of the 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.
Defence Secretary John Healey told the BBC early reports suggested it was an isolated incident.
Asked whether it was a terror attack, Healey said the investigation was ongoing and that police would “let us know as much as they can as soon as they can”.
He added that this was a service he used often – and that he had travelled along the same route just hours before the attack – and paid tribute to the emergency response.
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 she was receiving regular updates on the investigation and urged people to “avoid comment and speculation at this early stage”.
Conservative leader Kemi Badenoch told the BBC she was “horrified” by the attack: “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.
LNER, which operates East Coast Mainline services, urged passengers to avoid travelling on Sunday 2 November.
Ticketholders who are no longer planning to travel will be eligible for a full refund.
Unused weekend LNER tickets for Saturday or Sunday will be valid until Tuesday 4 November.
Customers who cannot avoid travelling on Sunday are advised to check their journey, as there may be cancellations or alterations.
Passengers can use their tickets on the following services without incurring extra cost:
• Avanti West Coast between London Euston, Manchester
• TransPennine Express between Manchester, Leeds and York/Newcastle
• ScotRail between Glasgow Central and Edinburgh Waverley
• Northern between Carlisle and Newcastle
• East Midlands Railway (EMR) between London St Pancras, Leicester and Sheffield.
• CrossCountry between Sheffield, Doncaster/Leeds, York and Newcastle/Edinburgh
• Greater Anglia between London Liverpool Street, Stevenage and Peterborough.
• Great Northern and Thameslink between London Kings Cross, Stevenage and Peterborough
• London Northwestern Railway services from Euston
LNER said delays were expected across the train system, including from other services and operators in the East of England and London, throughout Sunday.
Thameslink tickets that were not used on Saturday will be valid for Sunday.
Source: www.bbc.com