Their friendship began on July 17, 2014, with whispered secrets shared through the vent in the wall that separated their cells.

Jessica Burlew remembers the exact date because shed turned 17 the day before, the same day that Mariam Abdullah, then 16 and about to be charged as an adult with armed robbery, had been brought to Estrella Jail in Phoenix, Arizona.

When Abdullah arrived, shackled and belly chained, in the closed custody unit, where girls deemed incorrigible were held in their cells for 23 hours a day, Burlew had been there for about six months. In that time, Burlew said she hadnt had any contact with other teens and so was glad to hear Abdullahs voice.

I very much did consider Mariam a birthday present, Burlew, now 22, wrote from Perryville Prison in Goodyear, Arizona, where she is serving a 10-year sentence.

Over the months they were held in isolation, speaking through that vent, Burlew and Abdullah shared the things that teenagers do.

Abdullahs favorite color was red. She wanted to be a firefighter. Her favorite musical artist was Drake.

She was silly, rambunctious and seemingly carefree. Still, the isolation wore on her.

For the next two years, Abdullah spent much of her time separated from others, both to discipline her for what was classified as unruly behavior and to prevent her from harming herself. After she was transferred to Perryville in July 2015, she was in and out of solitary confinement in the minors unit for various infractions. Just after she turned 18, she was transferred to the Lumley Unit, the most restrictive unit in the prison where offenders were confined in their cells 23 hours a day, with one hour of recreation spent in a cage under the searing Arizona sun. When she was not in her assigned cell, Abdullah was in suicide watch cells, where again she was alone. Officers more than once carried her there by force.

After another woman in the prison, Cynthia Apkaw, died by suicide in August 2015, Abdullah wrote in a letter to a friend: To be honest with you being here makes me feel like that but I just havent acted on it yet.

On July 19, 2016, she did. A handwritten poem found in her cell after her death read in part:

In this place is a struggle for her

Shes alive but not living

Shes feeling everything, feeling nothing

Tired of existing and longing to join

Those who truly know peace

The dead.

HuffPost examined some 200 documents, videos and photographs from Abdullahs time behind bars, interviewed 16 people who knew her and reviewed her letters and other writings for this account of her life and death in isolation.

The almost 2.2 million people in correctional facilities in the United States are often amongst the most vulnerable, with troubles mental illness, addiction, social disenfranchisement that put them at a high risk of suicide before they even enter a cell. Incarceration exacerbates these problems; isolation makes a bad situation worse.

So much of who we are and how we function is implicitly dependent on interaction with other people. Its such a natural part of human life that its almost as second nature as breathing, said Craig Haney, professor at the University of California, Santa Cruz and an expert on the impact of solitary confinement. When all of that stuff is taken away, its destabilizing, psychologically unsettling, in a very profound way.

Self-harm and suicide rates in some state and federal correctional facilities have risen dramatically in recent years, an issue most recently highlighted by the death of financier Jeffrey Epstein , whose body was found in a federal facility in New York on Aug. 10. Shortly before his death, Epstein, who was awaiting trial on charges of sex trafficking, had been moved from suicide watch to a special housing unit.

Mental health experts and advocates say that the use of solitary confinement is a factor in many cases of suicide. The most recent nationwide data released by the federal Bureau of Justice Statistics shows that prisoner deaths by suicide in state prisons increased roughly 30% between 2013 and 2014 from 192 to 249 deaths.

At least 61,000 prisoners are in some form of isolation generally understood as being held in a cell 22 or more hours per day, for an average of 15 or more consecutive days in facilities across the country, according to a survey done by the Association of State Correctional Administrators and the Liman Center for Public Interest Law.

That reflects a drop nationwide, but the one statistic doesnt provide a full picture of what is happening in individual states.