The terror suspect accused of killing three people in a Reading park briefly came to the attention of MI5 last year, security sources have said.

Khairi Saadallah, a 25-year-old refugee who fled the civil war in Libya, was detained at the scene on suspicion of murder but was on Sunday re-arrested under the terrorism act.

Three other people were injured during the attack at 7pm on Saturday in Forbury Gardens in Reading town centre.

Prime Minister Boris Johnson said he was “appalled and sickened” by the attack and said “we will not hesitate to take action” if there are lessons to be learned from the circumstances.

The PA news agency understands that Saadallah briefly came to the attention of the security services in 2019, but the information provided did not meet the threshold for investigation.

A source said M15 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 was understood to be considered a major factor in the latest incident.

The suspect was jailed in October for a complex series of non-terror offences before his sentence was reduced to one of 17 months and 20 days’ imprisonment in the Court of Appeal.

One of the appeal judges who gave the judgment in March, Mr Justice Goss, noted Saadallah’s various mental health issues in reducing the sentence in March.

“The sentences were individually manifestly excessive and failed to reflect adequately the appellant’s mental health diagnosis and the principle of totality,” he wrote in the judgment obtained by PA.

Meanwhile, one of the victims of Saturday’s attack was named as 36-year-old history teacher James Furlong.

His parents Gary and Janet, said in a statement: “James was a wonderful man. He was beautiful, intelligent, honest and fun.

“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.”

Deliveroo driver Amir Hadyoon, 31, saw the suspect who was covered in blood arrested outside the job centre on Friar Street in Reading, a short distance from Forbury Gardens.

Mr Hadyoon told reporters the man had not resisted the officers, adding: “To me it looked very weird because he didn’t say a word. He didn’t care, he was just staring… he was just completely silent.

“He wasn’t even blinking, he looked really weird.”

Investigators have said they are not looking for anyone else in connection with the attack.

Chief Constable John Campbell, of Thames Valley Police, said: “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.

“We will be working closely with our partners over the coming days and weeks to support the Reading community during this time, as well as with Counter-Terror Police South East (CTPSE) as they progress their investigation.”

He asked the public to avoid the area and also to “avoid speculation” about the attack as he appealed for any video footage to be shared with police only.

“Out of respect for those deceased and injured, along with their loved ones, please do not circulate this footage on social media – this will be incredibly distressing.”

Footage of the aftermath of the attack in which the victims were receiving first aid appeared online on Saturday evening.

The Prime Minister has held a meeting with security officials, police and senior ministers over the incident.

In an interview in Downing Street, Mr Johnson said he is “appalled and sickened that people should lose their lives in this way and our thoughts are very much with the family and the friends of the victims today”.

“I’ve obviously talked to the chief constable at Thames Valley Police, thanked him and his officers for their bravery in tackling the suspect.

“We now have someone in custody. The police must get on with their job, get to the bottom of exactly what happened, and so it would be difficult really to comment in detail.

“Except to say this, if there are lessons we need to learn about how we handle such cases, how we handle the events leading up to such cases, then we will learn those lessons and we will not hesitate to take action where necessary.”

One of the patients admitted to Royal Berkshire Hospital has been discharged, while another remains in a stable condition under observation, a spokeswoman said.

Reading town centre was quiet on Sunday morning, with few people on the streets.

All roads surrounding Forbury Gardens – which is a short walk from the train station – remained cordoned off.

Blue and white tents have been erected next to the walls of the park, with officers patrolling a cordon.

The head of counter terrorism policing, Metropolitan Police Assistant Commissioner Neil Basu said the motivation for the killings are “far from certain”.

Speaking in London, he said: “Although the motivation for this horrific act is far from certain, counter-terrorism policing have taken responsibility for leading the investigation.”

He emphasised that the attack was not associated “in any way” with the Black Lives Matter protests that had taken place in the park earlier that day.

Mr 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”.

He said counter-terror police would be working “around the clock”, and thanked 41 witnesses who had come forward to assist with their inquiries.

Personal trainer Lawrence Wort, 20, who said he was around 10 metres from the attack, told PA on Saturday night: “The park was pretty full, a lot of people sat around drinking with friends, when 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.”

Floral tributes have been left by the park railings.

One read: “To the victims and their loved ones. You will be remembered. I proposed to Kayleigh here. I come here every Remembrance Day with Jack. It’s always been a special place.

“It’s so sad. Sorry for your loss. Ben and Kayleigh.”