See Hala Gorani's interview with Samy Amimour's sister on CNN International at 8 p.m. GMT on Friday.

Drancy, France (CNN) When Samy Amimour's older sister last spoke to him in August, it was an everyday conversation.

He told her to send kisses to the family and to his pet cat, whom he had left behind in France. He told her that he had a lot to deal with and that he'd call again soon.

But the next thing Anna Amimour heard of Samy was that he and two other men had stormed the Bataclan concert hall in Paris and shot dead 89 people before he blew himself up with a suicide vest.

"At first, I was shocked. I was screaming in despair and sadness," she said of the moment she heard of her brother's involvement in the November 13 attacks. "But when I gathered my thoughts, I thought, 'This must be wrong; there must be a mistake.' "

There was no mistake.

Samy Amimour, her 28-year-old brother, had been part of a coordinated terror plot that left 130 people dead and hundreds injured in the worst attack on French soil since World War II.

'Cheeky little boy'

In her first interview for international media, Anna told CNN of her family's shock and the guilt that she says "stings us on the inside."

Samy Amimour's sister remembers him as loving to laugh and joke

"We ask ourselves the question every day: What happened, why? He was in the same womb as me. We grew up together, so how did our paths end up so far apart?"

Anna still can't believe the little brother she remembers could be capable of such horror.

"He was a nice person, a sensitive person, a bit shy but somebody you can rely on. He was generous, someone who loved to laugh and joke, a cheeky little boy," she said.

That cheeky little boy turned into a mass murderer after traveling to Syria to join ISIS, where he's thought to have spent the past several years.

When he left, he told his family he was going to the South of France with friends. Little did they know he would never be coming back.

Sitting in the small apartment where she grew up with Samy, their parents and a younger sister, Anna says her brother's radicalization began on the Internet.

The family had never been strict in their religious practice, but they noticed Samy's behavior change about four years ago.

"The changes from the way he used to dress, the way he used to listen to music, the way he used to go out, the way he used to express himself -- to laugh and to think as well -- that can't happen overnight, not by himself," his sister said.

'Brainwashed' by recruiters

Samy had been working as a bus driver in Paris.

Anna recalls proudly that he'd scored the second highest result in the exam for the RATP, the city's public transport network.

She says Samy was targeted by recruiters in their neighborhood of Drancy, a drab suburb on the outskirts of Paris. Anna believes he was brainwashed.

"They came to talk to him more and more and told him that he should attend the sermon at the mosque more regularly and that he should be devoted to his practice of Islam. Then they led him towards mosques that were more radical," Anna said.

Samy's radicalization came to the attention of French security services in 2012, after he attempted to travel to Yemen and was charged for "activities in collaboration with a terrorist enterprise."

No help

The family says they tried to get help.

"We tried to sound the alarm. We talked to people around us and tried to knock on doors, asking for help," Anna said.

Photos: Paris attack suspects: What we know Photos: Paris attack suspects: What we know Hide Caption 1 of 10 Photos: Paris attack suspects: What we know Hide Caption 2 of 10 Photos: Paris attack suspects: What we know Hide Caption 3 of 10 Photos: Paris attack suspects: What we know Hide Caption 4 of 10 Photos: Paris attack suspects: What we know Hide Caption 5 of 10 Photos: Paris attack suspects: What we know Hide Caption 6 of 10 Photos: Paris attack suspects: What we know Hide Caption 7 of 10 Photos: Paris attack suspects: What we know Hide Caption 8 of 10 Photos: Paris attack suspects: What we know Hide Caption 9 of 10 Photos: Paris attack suspects: What we know Hide Caption 10 of 10

She declined give details about who the family approached, and she says they didn't get any help.

After he was charged in 2012, Samy was placed under judicial supervision, requiring him to check in regularly with authorities.

But he violated the terms of his supervision and then became the subject of an international arrest warrant in 2013. By that point, he was believed to be on his way to Syria.

According to an interview with French newspaper Le Monde, Amimour's father traveled to ISIS-controlled Syria in June 2014 in an attempt to persuade his son to return.

Anna refused to confirm that the trip took place.

In Le Monde, Samy's father describes his encounter with his son as cold: He had a "distant sort of smile." Samy had taken on the nom de guerre Abu Hajia and had married. His father's mission to bring Samy home to France failed.

A year and a half later, the Amimour family is struggling to come to terms with what Samy became.

"Of course there's part of us that says maybe it's our fault, maybe we could have done something different. Maybe, just maybe," Anna worried.

But she knows her family is far from the only one suffering. She understands that her words won't help the victims' relatives: "We understand the pain they feel, and we know nothing will bring their families back, whatever we may say."