Police have declared the stabbing attack in Reading in which three people died a "terrorist incident".   A 25-year-old man from Reading was arrested at the scene of the attack at about 19:00 BST on Saturday, which also left three people seriously injured. Counter-terrorism police are now taking over the investigation, Thames Valley Police said. Security sources said the man arrested is thought to be Libyan. In a statement, Thames Valley Police said the attack had been declared a terror incident by Deputy Assistant Commissioner Dean Haydon, senior national co-ordinator for the Counter Terrorism Policing network. Chief Constable John Campbell said it was a "truly tragic incident". "Incidents of this nature are very rare, though I know that will be of little comfort to those involved and understand the concern that this incident will have caused amongst our local community," he said. Speaking on the BBC's Andrew Marr Show, Mr Hancock said Home Secretary Priti Patel and Prime Minister Boris Johnson would be in contact with police leading the investigation later on Sunday. "It's clearly a terrible crime whatever the motivation and we will be making sure the police have all the support they need," he said. The prime minister met security officials, police and senior ministers at Downing Street on Sunday at 09:00 BST and was updated on the ongoing investigation. An eyewitness to the attack at Forbury Gardens, a park near the centre of Reading, said he saw a man moving between groups of people in the park, trying to stab them. Three people died in the attack, and another three were seriously injured. The arrested man had previously been in prison in the UK for a relatively minor conviction, not a terrorism offence, sources said.