A teenager alleged to have stabbed the wife of an American professor to death in Russell Square had made three attempts to take his own life in the past six months, family friends said today.

Zak Bulhan, 19, was today being questioned over the “spontaneous” six-minute rampage at 10.30pm on Wednesday which left five other people suffering stab wounds.

More than 30 police officers searched the council flat where Somali-born Bulhan lives with his mother and two siblings next to St George’s Hospital in Tooting at 6.30am yesterday.

The IT student, who moved to the UK aged five in 2002 after his family claimed asylum in Norway, had just completed his first year studying for a B-Tech at South Thames College’s Morden campus.

Friends were today in shock that the Chelsea fan and “teacher’s pet” had been arrested in connection with the bloody attack.

He was today being questioned on suspicion of murder as tributes were paid to victim Darlene Horton, 64.

The retired teacher was in London with her husband, psychology professor Richard Wagner, as part of Florida State University’s summer study programme.

The couple had been due to return to their home in Tallahassee last night.

Mrs Horton was stabbed in the back with a kitchen knife. Her last words were ‘he’s still here, he’s still here’ as she tried to warn others of the danger.

Armed police took six minutes to reach the scene and detain the knifeman with a Taser.

He spent the night in hospital before being transferred to police custody where he was today being questioned on suspicion of murder.

Scotland Yard said it no longer believes the mass stabbing was terror-related, and detectives are instead focusing inquiries on whether the suspect had mental health issues.

A close family friend told The Times that Bulhan had called an ambulance three times in the past six months.

She added: “He has been very unwell. He wanted to kill himself. I saw his mother with an ambulance outside their flat and she said Zac had called it because he wanted to hurt himself.

“He’s called the ambulance about two more times because he was feeling unwell. His mother was very afraid.”

Former friends at Graveney School in Tooting, described Bulhan as a “teacher’s pet” who was occasionally bullied during his his early years.

A former school friend said: “I can’t believe it, I’m just telling myself it must be a mistake.

“We used to get really competitive over grades in maths and debate religion all the time.

“He was a devout Muslim and he would passionately defend it, but he respected my opinion too. Unless he was somehow brainwashed I don’t know how it could be the same guy.

“He was a bit socially awkward but as far as I knew he didn’t have mental health problems.

“He wasn’t the jock but he wasn’t the kid who ate glue at the back of the class. He just flew under the radar.”

Another student from his school year said: “He was picked on a bit when we were younger, people used to bully him about the shape of his head and silly things that kids do. I remember him getting really down about it at first but he got over it.

“He seemed to have a good life with his mother. He didn’t get into drugs or petty crimes like the some of the other kids.

“We all split up going to different colleges and he was never big on social media so we lost touch.

“His friends told me he once said he ‘wanted to stab them all’. When they called him up on it he backed down saying he was only joking, but it was an odd thing to say.

“This is the first I have heard of him having mental health problems.”

Neighbour Jennifer Ocampo, 31, a carer, said the family were “best neighbours you could hope to have”.

She said: “He was the quiet type and he wouldn’t make eye contact if he passed you on the stairs, but his mother is really nice.

“She always wore a full black burka but her sons wore Western clothes.”

As well as killing Ms Horton, the knifeman also injured five others including Israeli holidaymaker Yovel Levkowki, 18, who was slashed across the hand as she approached him to offer help after believing he was a victim.

She said she was lucky to be alive after watching the attacker go on to slaughter Ms Horton in front of her eyes.

“I don’t know if it’s luck or fate but it’s a miracle,” she said. “The woman who was standing next to me died before my eyes.”

Police said the other victims include a British man who remains in a serious but stable condition after being stabbed in the stomach.

An American man, who suffered chest wounds, an Australian woman who was stabbed in the back and an Australian man who was stabbed in the chest, were all discharged from hospital yesterday.

Paying tribute to Mrs Horton, FSU President John Thrasher said: “There are no words to express our heartache over this terrible tragedy.

“We are shocked that such senseless violence has touched our own FSU family, and we will do all we can to assist Professor Wagner and his loved ones, as well as his friends and colleagues in the psychology department, as they mourn.”

Mark Rowley, the Met’s assistant commissioner in charge of counter-terrorism, said: “Whilst the investigation is not yet complete, all of the work we have done so far increasingly points to this tragic incident as having been triggered by mental health issues.

“At this time, we believe this was a spontaneous attack and that the victims were selected at random.”