In her letter to Postmedia News, published in today's edition of the National Post, Susan Bibeau said her estranged son had wanted to travel to Saudi Arabia to study Islam and the Koran, not Syria to join militant fighters as stated by police.
But when his passport application was repeatedly denied, Zehaf-Bibeau feld trapped, "unable to stay in the life he was in, unable to move on to the next one he wanted to go to," his mother wrote.
She stressed that her son suffered from great psychological problems and was not the terrorist that some described, while at the same time condemning his murderous act.
"As a person and mother I am horrified by the actions of my son, I am sickened by it. I will never understand what drives a person to such senseless violence," Bibeau wrote.
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She added that her son's acts were so horrific that she had buried her sadness and mourning "deep within me."
Royal Canadian Mounted Police Deputy Commissioner Mike Cabana acknowledged to the Post that earlier police statements that Zehaf-Bibeau had intended to travel to Syria were incorrect.
But Cabana said the RCMP saw no need to correct the record since militants traveling to Syria often first pass through Saudi Arabia or Turkey.
"Most will call my son a terrorist. I don't believe he was part of an organisation or acted on behalf of some grand ideology or for a political motive," Bibeau wrote.
"I believe he acted in despair... I believe his actions did create terror."
But more than any ideology, Bibeau said her son was in the grips of serious mental illness that deteriorated with his use of drugs.
His mother also mentioned his alienation from the home, recalling that he preferred to stay in homeless shelters than return to her house.
"For me mental illness, is at the center of this tragedy," Bibeau wrote.
"At some point in his life, my son had a serious addiction to drugs, I don't know if he overcame it, but doing so much of it could have left permanent marks and led to his current mental state."
She expressed deep apologies to the Canadian people as a whole and in particular to the family of the soldier who was killed, corporal Nathan Cirillo.