When Trey describes how she, her mother, and her sister lived in constant fear after members of a drug cartel killed her father and uncle in Mexico, she remains composed.

Worried they could be next, Trey — then 13 years old — and her family fled to Toronto, Canada in 2004 and filed for refugee status, hoping for the chance to start a new life.

After their claim was denied and the government issued an order for their deportation, Trey and her family flouted it and went underground.

Even though it was safer than Mexico, they were overcome by anxiety. Everyday, she prepared herself for when border agents would come knocking on the door.

Early one Saturday morning in 2008, when Trey was 17, Canada Border Services agents showed up after someone reported their status. The officers handcuffed Trey's mom and forced them into an unmarked car to take them to the immigration holding center in Toronto.

This is the part of the story that brings tears to Trey's eyes. "It was even worse than I thought it would be," Trey, who is using a nickname, told VICE News. She describes how she and her mom were assigned numbers and had to stay in a room with two beds, constantly watched by guards, and not knowing what would happen next.

"It was just a plain room, but I knew it was a jail. I had no idea if I would ever leave," she remembers.

Trey says she was let out five days later, her mother followed the next week. After a lengthy immigration process, they now have permanent resident status in Canada.

But those five days still haunt Trey. "It was as if we had committed the worst crime ever, like we had killed someone. I was treated more like a number than a person," she said. "But the only thing we had ever done was try to survive."

For Rachel Kronick, a researcher at Toronto's Center for Addiction and Mental Health who specializes in child psychiatry, Trey's situation isn't surprising. Kronick's new study on children in Canadian immigration detention centers, in the current issue of the _American Journal of Orthopsychiatry (_which is the study and prevention of behavioral disorders among children), has found that the detention of migrant children, even for very short periods of time and in relatively safe conditions, is damaging to their mental health. This is especially so among those who experienced trauma before they got there.

"I'm hoping that the study will shine a light on what's actually happening to vulnerable children and migrants in Canada. It's been an open secret and these people are truly voiceless," Kronick told VICE News. "I can say from a mental health perspective that it's not okay for children to be detained at all."

From 2005 to March of this year, approximately 4,392 children have been held in Canadian immigration detention centers. However, Kronick says this number could be two or three times higher because the government does not keep count of children who are not detained officially, including Canadian citizens, who are seen as "accompanying" their parents.

Kronick's study — the first time researchers have had access to detained children in Canada — focused on 12 families she interviewed, in 2011 and 2012, while they were held in the immigration holding centers in Toronto and Laval and eight families who had been released from detention. One family was interviewed both during and after they were detained.

Kronick observed troubling symptoms among the children who lived there, ranging from sleeplessness to anxiety and post-traumatic stress disorder. "What we saw was that even children detained for two days developed such severe anxiety symptoms. In one case, a boy who was eventually released, wasn't able to go to school for weeks," she said.

Canadian border officials can detain asylum seekers and permanent residents if their identity is in question, when they are suspected of not showing up to their next hearing, or if they are deemed a safety threat. According to its website, the CBSA detains children under 18 "only as a last report" and their best interests will be "carefully considered." CBSA has said it does not comment on third-party studies.

Most detained migrants, including all families with children, are held in one of Canada's three designated immigration holding centers, located in Toronto (195 beds), Laval (150 beds), and Vancouver (24 beds for a maximum of 72 hours). Detained migrants without children may be held in provincial jails across the country.

Last week, VICE News reported on a new study from the University of Toronto that criticized CBSA's treatment of adult migrants with mental health issues and called for the creation of an ombudsperson to oversee its activities.

Unlike the US and UK, there is no cap on how long migrants can be detained in Canada. One child, who was born in Canada, has lived with his asylum-seeking mother from Cameroon in the Toronto immigration detention center for two years.

Other countries are considering new ways to handle immigration cases involving children. The UK recently started referring them to social services after 24 hours, and a federal court in the US will soon decide if the government's policy to detain migrant families is legal.

Similar to prison, life for families in Canada's immigration holding centers is highly regulated. Mothers and children are separated from fathers and husbands, and they can visit with each other for 15 to 30 minutes per day, according to the report.

Eating and sleeping must happen at certain times and detainees need permission to go outside (which can happen only twice a day) or enter another section of the building. Security cameras and security guards watch the detainees' every move.

"The pervasive under-stimulation and constant surveillance of the children and of their mothers transformed daily life into an experience of deprivation and powerlessness," the report says.

Because of overcrowding in one detention center, one mother had to share a cot with her 12-year-old son for almost one year.

The part of her research that struck Kronick most is the tense and often contradictory relationship between the children and the security guards

"Children and their parents experience the surveillance in detention as invasive, frightening, and criminalizing, even as it is marked with individual guards' attempts to care for families," she writes in the report.

In some instances, guards would bond with the children and bring them gifts and used clothing. One guard reportedly referred to babies as "my love" or "my baby."

One mother told Kronick a guard had agreed to watch her baby while she napped, going against a policy that forbids guards from looking after the children.

Most families reported lasting effects from their time in detention, including delays in their children's social and educational development. Children continued to be distressed, some stopped talking for weeks, and others feared being separated from their parents.

One seven-year-old who was detained refused to attend school because he was scared of being "taken away" back to detention. He was also fearful of police officers and vans, which reminded him of the one CBSA had forced his family into.

She concludes that Canada should halt its practice of detaining children entirely.

"I wish Canadians could think of these children as their children," Kronick said. "Many of these people will go on to become Canadian citizens, and for these people to have their first experience in Canada be one of traumatization is not healthy for the future of our society."

_Follow Rachel Browne on Twitter: _@rp_browne

Watch the VICE News Documentary, 'Immigrant America: The High Cost of Deporting Parents.'