Horror struck the home counties in England on Saturday night (UK time) after three people were stabbed to death and two others were left in a critical condition following a frenzied broad daylight attack at a park in Reading town centre.
Sources told The Sunday Telegraph it was understood a suspect arrested at the scene was Libyan.
UK Prime Minister Boris Johnson said: "My thoughts are with all of those affected by the appalling incident in Reading and my thanks to the emergency services on the scene."
According to eyewitnesses, a man began screaming and then produced a knife and embarked on a frenzied stabbing spree shortly after 7pm in Forbury Gardens, a park in the centre of Reading, Berks.
Lawrence Wort, 20, a personal trainer from Chippenham, watched the horror unfold from a distance of no more than 30 feet away.
He said the attack appeared "completely random"with no telltale clue, no previous tension that might have suggested what was about to happen.
"The park was pretty full, a lot of people sat around drinking with friends," said Mr Wort.
"One lone person walked through, suddenly shouted some unintelligible words and went around a large group of around 10, trying to stab them. He stabbed three of them, severely in the neck, and under the arms, and then turned and started running towards me, and we turned and started running.
"When he realised that he couldn't catch us, he tried to stab another group sat down, he got one person in the back of the neck and then when he realised everyone was starting to run, he ran out the park."
South Central Ambulance Service NHS Foundation Trust said a huge medical response was sent to the scene, including five ambulance crews, a hazardous area response team, critical care response unit from Thames Valley Air Ambulance and teams from the London Ambulance Service.
In a statement, it added: "We were assessing and treating a number of casualties who had sustained injuries as a result of the incident. Thank you to all those people who stopped and helped those who were injured as a result of this situation."
The attack came hours after a Black Lives Matter march was held in the same park.
Police said there was no link to the demonstration.
Priti Patel, the Home Secretary, said she was "deeply concerned to hear of reports of an incident in Reading."
She added: "My thoughts are with everyone involved including police and emergency responders at the scene."
Ms Patel was kept updated throughout the night by police as the situation developed.
Sources told the Telegraph that three people had lost their lives at the scene.
Videos emerged online of the aftermath showing police officers, paramedics and members of the public desperately trying to save the lives of those who had suffered severe stab wounds lying unresponsive.
Thames Valley Police said: "Officers attended along with the ambulance service and a number of people were injured and taken to hospital.
"Tragically, three of these people died, and another three sustained serious injuries.
"A 25-year-old man from Reading was detained at the scene and has been arrested on suspicion of murder. He is currently in police custody."
Detective Chief Superintendent Ian Hunter, Head of Crime, said: "The Thames Valley Police Major Crime Unit has launched a murder investigation following this incident.
"We have made one arrest and at this stage we are not looking for any other people in connection with this incident. There is no intelligence to suggest that there is any further danger to the public, however we urge people to remain vigilant and report anything suspicious by calling police.
"This is not currently being treated as a terrorism incident, however officers are keeping an open mind as to the motivation for the incident and are being supported by colleagues from Counter Terrorism Policing South East."
Local businesses and residents reported huge swathes of the town centre, including a retail park, were ordered into lockdown as force helicopters hovered and police vans and squad cars arrived at the scene.
However, it remained unclear whether it was feared any accomplices may be at large.
Nieema Hassan, one of the organisers of the peaceful demonstration, said: "In terms of the protest and the people who attended from Black Lives Matter, we're all safe."
"None of us are affected. We had all left by the time this happened." Two air ambulances landed at the nearby Kings Meadow to fly two patients to the Royal Berkshire Hospital. The leader of Reading Council Jason Brock tweeted:
"Concerning reports from Reading town centre – please stay clear of the area as Police are dealing with a serious incident."