Anders Behring Breivik's best friends thought he was either depressed or gay when he stopped seeing them in 2006 - five years before he killed 77 people in Norway.
At his trial in Oslo, 33-year-old Breivik has admitted responsibility for a bomb blast that killed eight people in the Norwegian capital and shooting dead 69 people on Utoya island last July.
But he has pleaded not guilty to murder, claiming he acted in self-defence.
The trial will decide whether he receives a prison sentence or is declared legally insane and sent to a psychiatric facility.
In 2006, Breivik moved back to his mother's flat and islolated himself in his room. In reality he was preparing for one of the worst terror attacks to be carried out in Europe.
His best friends decribed Breivik as "normal", "social" and even "popular", but he changed and withdrew to his bedroom in his mother's flat.
"He stopped replying to text messages and when we knocked on his door on his birthday, his mother told us he didn't want to see us. We thought this was odd," his 33-year-old former flatmate said.
He was one of three friends who reluctantly turned up to Oslo district court to give evidence about Breivik's upbringing.
They all refered to their former friend by his first name, but none of them want to be named.
They also tried to stop their evidence being broadcast to 17 other district courts, but their request was denied by the judge.
Breivik's former friends refused to give evidence infront of the far-right extremist, so he was removed to a back room together with two of his lawyers and two forensic psychiatrists watching his reactions during their evidence.
Some of his friends said they were convinced Breivik was gay and thought this was the reason why he did not want to see them.
He was concerned about his looks more than the average person, his friends told the court. Some even described him as "feminine".
Breivik had a nose job at the age of 20 to get a more "Aryan" nose, it was claimed, and he wore make up and sunglasses when they went out in the evenings.
One friend said: "We thought this was odd."
Another friend was convinced Breivik was depressed, as he stopped looking after himself and looked "tired".
"He started to isolate himself. This wasn't like him. He was normally very social. I thought it was strange when he moved back to his bedroom in his mother's flat."
In his bedroom Breivik played the computer game World Of Warcraft night and day, and he adopted more radical views and right-wing thoughts on immigration.
The 27-year-old ex-girlfriend of Breivik's best friend told the court how she thought Breivik had become ill from his gaming addiction and thought he should get help.
Prosecutor Inga Bejer Engh asked the woman why her boyfriend never gave up trying to get Breivik out into fresh air and daylight.
"Because he was a good and faithful friend," she replied, weeping in court.
Like other friends she described a social and interesting friend who changed after 2006 when he became more political, and started saying he was afraid that Islam would take over Norway and Europe.
When he met up with friends on rare occasions, he started using more complex and foreign words.
In his bedroom, isolated from his friends, he wrote his manifesto and planned the worst terror attack Norway has ever experienced.
Having spent four years with hardly any contact with his friends, Breivik again started seeing his friends in 2010, attended BBQs and dinner parties.
"Good old Anders was back," a friend said.
One of them met him just before the terror attacks and others had arranged to meet him at his farm, where he built the Oslo bomb, the week after his July 22 attack.
The trial continues.
