She says she is in shock that her son turned out to be one of the killers, and she hurts for the survivors of the victims, struggling to articulate what to say to them..
"There are no words," 68-year-old Valeria Khalija Collina told CNN.
"He met the wrong people," she said in a CNN interview in Fagnano, near Bologna in northern Italy. "I don't know what they did to him.
On a security watch list
Zaghba, 22, was one of three men who rammed a van into pedestrians on London Bridge before launching a stabbing spree in bars and restaurants at nearby Borough Market on Saturday night. He and the two others were shot dead by police.
Eight people died and dozens were injured in the attack. Asked what she would say to the victims' families, Collina said, "I feel ashamed when I try to think of words.
"I feel the same feelings they do. And I know what they are feeling. More than words, I would like to show my commitment to what I can do to try make sure these kinds of things never happen again... with all my strength," she said.
A Moroccan-Italian, Zaghba had been put on a security watch list, police in Italy said..
He was stopped at the Bologna airport by Italian airport police in 2016 as he was trying to board a plane bound for Istanbul.
Police found what they described as extremist material on his cellphone.
But they weren't able to arrest Zaghba, who was traveling on an Italian passport, because having extremist material on a phone isn't against the law in Italy.
Italian media reported that officials suspected he was bound for Syria and that Italian authorities had alerted their British counterparts about his movements.
"In March 2016, he was visiting me in Italy and told me that he wanted to travel to Rome. But he was stopped by the police at the Bologna airport because of his suspect appearance and he only had a small backpack and a one way ticket to Istanbul," Collina said.
"The police then took his Italian passport, but they gave it back in April 2016 because they had no legal reason to keep it."
'Looking for rules and boundaries'
Collina said her son traveled to London for the first time in the summer of 2015 and that in September of that year, he returned to Morocco to finish his third year of university in Fez, where he was studying engineering and computer science.
This year, she said, he was an intern at Eman Channel in London, working as a sound technician at the TV station. He eventually got a six-month contract with the station, the mother said.
The channel's goal is "to produce high quality programmes, in terms of both production value and Islamic principles," according to its Facebook page.
"As soon as his name was circulated by the media, as a suspect, in the terrible attacks witnessed in London, we immediately contacted the police. We are now fully cooperating with all enquiries,' Eman Channel said Wednesday in a statement.
"At no time, during his work at the Channel, did he express extremist thoughts or display any sympathy towards extremist organisations abroad," the statement continued. "Our thoughts and prayers go out to the victims of this terrible attack and as part of our broadcast agenda we continue to encourage communities to report anything suspicious to the police."
"I was proud of him. It was like being promoted. He was a hard worker and also did extra work at the gym where he used to lift weights," Collina said.
She said Youssef was loved by both his Italian and Moroccan families.
"He was a sweet boy that was looking for rules and boundaries." He had a girlfriend in high school and was trying to understand the boundaries of the religion and his feelings for her, she added.
Seeking a perfect 'Islamic state'
Although her son's ideological bent became apparent, Collina said he never showed hate for Western society.
"After he was stopped at the Bologna airport, he told me he wanted to go to Syria to live in a perfect Islamic state, work there and raise a family," she said.
Once in Morocco, she said, Zaghba showed her an ISIS video.
"Everything appeared perfect. Those depicted were distributing food to the populations, dams and roads were being repaired," she said.
Her son called her on the Thursday before the Saturday night attack, she said. He usually sent messages instead of giving her a ring.
That conversation, she realizes now, "was his farewell call." She wept and couldn't speak when she recounted their back-and-forth.
"He had a sweet, almost melancholy voice. We talked about silly things," she said.
"He moved into a furnished garage in the garden. And I joked with him, asking him, 'is there a toilet?' "And he said 'no, I go through the garden to get to the house.' But he said, 'it's very nice. I open the door and there's the garden.' "
In retrospect, she had an epiphany.
"Afterward, I connected the image of the garden to the paradise described in the Quran. And my thought was that is what he was looking for."