A terror suspect held over the knife attack in a Reading park that killed three people came to the attention of MI5 last year, security sources have said.
The 25-year-old man was detained a short distance from the scene at Forbury Gardens and arrested on suspicion of murder.
He was later re-arrested under Section 41 of the Terrorism Act, which gives police the power to detain him without charge for up to 14 days.
According to PA Media, the suspect briefly came to the attention of MI5 last year, but the information provided did not meet the threshold of investigation.
PA understands from security sources that MI5 had received intelligence he planned to travel abroad, possibly for terrorism purposes, but the threat was found to be insubstantial.
As counter-terror officers investigate, mental health is understood to be considered a major factor in the latest incident.
Police have said they are not looking for anyone else in connection with the attack.
School teacher James Furlong, 36, is the only victim to be named by police so far.
However, Joe Ritchie-Bennett has been named by friends and family as a second victim.
His father Robert Ritchie told US TV network CBS that he had been caught up in the attack in Forbury Gardens.
In a tribute, Ritchie said: “The family is heartbroken they have lost their brilliant and loving son. This was senseless.”
Two other people injured in the attack remain in hospital, while one has now been discharged.
Boris Johnson said he was “appalled and sickened” by the incident and said “we will not hesitate to take action” if there are lessons to be learned from the circumstances.
Despite treating the incident as a terror attack, Metropolitan Police Assistant Commissioner Neil Basu said on Sunday the motive is still “far from certain”.
Basu said there was no evidence to suggest anyone attending crowded places is at risk, but asked people to “be alert, not alarmed, when you are in public”.
Security guard Sydney McDonald, 65, who saw the suspect being rugby-tackled to the ground by police, described how it looked as if he had put his hands “in a big bucket of red paint”.
He told the PA news agency: “I had just finished work at about 7.10pm and as soon as I came out of the shop, there were about four police cars, they turned around in the middle of the road, they were driving pretty fast.
“There was a guy and I saw him pointing to a man and saying ‘There he is, there he is’. If he hadn’t, they would have missed him. He was running really fast, properly fast.
“They put the emergency brakes on, jumped out of the car and rugby-tackled him to the floor. He was on his stomach and the blood on his hands looked like he had put his hand in a big bucket of red paint.
“They put the handcuffs on, he wasn’t putting up a fight or anything like that, they picked him up and put him in the van, he just sat there all quiet, he wasn’t saying nothing.”
Personal trainer Lawrence Wort, 20, who said he was around 10 metres from the incident, told PA on Saturday night the suspect was alone and “shouted some unintelligible words” before launching the attack.
“He went around a large group of around 10, trying to stab them,” he said.
“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.”
Thames Valley Police have pleaded with the public not to share any pictures or images of the killings on social media, and instead contact them on 101, or upload the footage to UKPoliceImageAppeal.co.uk.
Tributes pour in for victim James Furlong
Tributes from former students of Furlong poured in after a letter from the two co-head teachers at his school – the Holt School in Wokingham – announcing his death was circulated online.
His parents, Gary and Janet, said in a statement: “He was the best son, brother, uncle and partner you could wish for.
“We are thankful for the memories he gave us all. We will never forget him and he will live in our hearts forever.”
One former student wrote on Twitter: “James Furlong was one of the most influential and exceptional people in my life. He fostered my love of history, nurtured me, and taught me resilience and kindness.
They added: “The four years I spent with him were precious, and deeply treasured. I am absolutely devastated.”
Meanwhile, one of his colleagues called him a “kind and giving man”.
“He was an inspirational mentor for new teachers,” they wrote. “I learnt something from him every time we chatted about history teaching. This is so so unfair.”
Furlong’s former student Molly Collins said that pupils at the Holt School would light candles and lay flowers in his memory at a local church, while others were putting together a memorial book for the teacher’s family.
