Counter-terrorism police have joined the investigation after a stabbing attack in Reading left three people dead and two critically injured. Thames Valley said officers and other emergency services were called to reports of “multiple stabbings” at a park in Forbury Gardens, Reading at 7pm Saturday evening. One man, who is understood to be Libyan, has been arrested. Police said several casualties were taken to the Royal Berkshire hospital. A hospital spokeswoman confirmed that two people were in a critical condition. Disturbing videos on social media showed scenes of victims receiving resuscitation treatment. Shortly after the stabbings initial inquiries led to police calling in counter-terrorism investigators. Inquiries are underway to establish if terrorism may have been a motive but too early to be definitive yet. Early inquiries by counter-terrorism investigators are pointing in that direction, so counter-terrorism police and the security service, MI5, are assisting the investigation. After the stabbings police raided a block of flats in Reading. Photos from the scene of the raid showed that counter-terrorism specialist firearms officers were present. Senior government officials including Boris Johnson, the prime minister, and Priti Patel, the home secretary, are being kept informed of developments. Suggestions that it was linked to a Black Lives Matter protest that took place in the park earlier on Saturday have been dismissed. Organisers said the peaceful protest ended nearly two hours before the incident at 7pm. Crowds who attended had dispersed without any trouble, a spokeswoman said. Thames Valley police said: “There is no indication that this incident is linked to the Black Lives Matter protest that took place in Reading today.” Two air ambulances and several police vehicles responded to the incident. Johnson tweeted: “My thoughts are with all of those affected by the appalling incident in Reading and my thanks to the emergency services on the scene.” Patel said that she was “deeply concerned” by the developments. Freelance journalist Claire Gould, who lives near the scene, told the PA news agency she saw emergency air ambulance vehicles land on the nearby Kings Meadow as a police helicopter circled overhead. She described hearing lots of sirens and said surrounding roads and a retail park had been cordoned off by officers. 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.” “Officers are on the scene and investigating the incident.” The National Police Chief’s Council tweeted: “Please avoid speculation or sharing of video or images of the serious incident in Reading. An investigation will be ongoing.” Keir Starmer, the Labour leader, described the stabbing as “very concerning” and urged people to listen to the NPCC and not share footage of the incident that has been circulated online. In November the UK’s national terrorism threat level was lowered from severe to substantial for the first time in five years. The “substantial” threat level means there is a high level of threat and an attack might well occur without further warning. Since the UK terrorism threat was lowered, there have been two jihadist attacks. Weeks after the official announcement, on 29 November, Jack Merritt, 25, and Saskia Jones, 23, were stabbed to death by Usman Khan, 28, who was tackled by people at a prisoner rehabilitation conference at London Bridge. Khan’s attack was halted only when he was shot dead by armed officers. Then on 2 February, Sudesh Amman, who had been recently released while serving a terrorism sentence, stabbed two people in Streatham, south London. He wore a fake suicide vest and armed surveillance officers had been following him and shot him dead.