Protesters outside the Waller County Jail in Hempstead, Tex., where Sandra Bland was held until her death. (Melissa Phillip/Houston Chronicle via AP)

Controversy and questions continue to surround the death of Sandra Bland, a 28-year-old African American woman found hanging in her cell earlier this month. Local authorities ruled her death a suicide, while her family and friends said it was “unfathomable” that she would have taken her own life.

Even as Bland’s death has created a national firestorm, with the traffic stop that preceded her arrest drawing heavy criticism, this situation also points to another reality of the U.S. criminal justice system: The staggering number of inmates who die behind bars each year.

An average of about a dozen inmates die each day, according to the Justice Department. About 4,400 jail and prison inmates die every year, federal statistics show, a tally that does not include executions (which are infrequent, numbering in the dozens each year and adding up to just a fraction of all other inmate deaths).

Most of these deaths occur in state prisons, where 3,351 inmates died in 2012, the most recent year for which data is available, according to the Bureau of Justice Statistics. State prisons have the largest inmate populations, accounting for a little more than half of all inmates in custody that year.

[Prosecutor on Sandra Bland autopsy: No evidence ‘to indicate that this is a homicide’]

In local jails, 958 inmates died in 2012, but the causes of these deaths were very different from the causes cited in state prison deaths. Suicide is the leading cause of death for inmates in local jails, accounting for nearly a third of all deaths. (Heart disease is the second-leading cause of death.)

Suicides are cited as the cause far less often in state prisons, where cancer is the leading cause of death. Heart disease and other illnesses are listed as the cause in most other deaths.

Deaths in local jails tend to occur relatively quickly after the inmates arrive. About one in three suicides takes place within a week of the inmate’s arrival; more than half of these deaths occur within the first month. More than 70 percent of the inmates who died in local jails were not convicted at the time they died.

[Prisoners are much more likely to die in jail than escape]

Why does this happen? In 2010, the National Institute of Corrections released a report on jail suicides, outlining possible explanations for the problem. In short, they said, jails tended to exacerbate suicidal behavior:

Experts theorize that two primary causes for jail suicide exist: (1) jail environments are conducive to suicidal behavior and (2) the inmate is facing a crisis situation. From the inmate’s perspective, certain features of the jail environment enhance suicidal behavior: fear of the unknown, distrust of an authoritarian environment, perceived lack of control over the future, isolation from family and significant others, shame of incarceration, and perceived dehumanizing aspects of incarceration. In addition, certain factors are prevalent among inmates facing a crisis situation that could predispose them to suicide: recent excessive drinking and/or drug use, recent loss of stabilizing resources, severe guilt or shame over the alleged offense, current mental illness, prior history of suicidal behavior, and approaching court date. In addition, some inmates simply are (or become) ill equipped to handle the common stresses of confinement.

The report goes on to note that to prevent suicides, personnel need additional training, constant observation for people on suicide watch and a better screening process. In Texas, the screening process has come under fire as officials released documents with contradictory answers to the question of whether Bland had attempted suicide before, confusion that a Waller County assistant district attorney said likely explained the decision not to place her on suicide watch. The World Health Organization says that communication among different staff members is crucial, adding that additional screening and observation are also necessary.

[Report: While in jail, Bland left a voicemail saying she was at “a loss for words"]

Beyond suicide, thousands of people are dying behind bars each year, as the prison population has exploded over the last few decades.

There are about 2.2 million people incarcerated in jails and prisons, a number that is more than four times the population in 1980. Harsh sentences in the 1980s and 1990s helped foster a situation where an increasingly large number of inmates are older than 50, and state health-care spending on inmates has skyrocketed. The federal prison population has similarly climbed.  In state prisons, a little more than half of inmates were sentenced for violent crimes; one in five is there for property crimes like burglary, while about one in six is being held for drug crimes.

All told, one of every 108 adults in the United States was incarcerated in a prison or jail in 2012, according to the Justice Department. That same year, one in 50 adults were on probation or parole.

Related:

The fastest-growing population in federal prisons? People age 50 and older.