Upon his death, Hernandez, who has been accused of three murders, became something else: One of the growing number of people who take their own lives while behind bars.

Suicide has long been the leading cause of death for inmates in local jails, according to national statistics gathered by the Justice Department. In state prisons, where inmates are typically held for longer sentences, most inmate deaths are due to illnesses, the national statistics show, but the number of inmates taking their own lives has sharply increased.

The overall number of inmates who died in jails and prisons went up in 2014, according to the most recent data from the Bureau of Justice Statistics, which collects it from state corrections department and jail jurisdictions.

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In state prisons, the number of suicides went up 30 percent in 2014 from the previous year, increasing to 249 deaths from 192 in 2013. While nearly nine in 10 prisoner deaths in 2014 resulted from illnesses, suicides that year accounted for about 7 percent of the deaths, the highest percentage in a year since 2001.

Local jails, where suicides have long been reported to be the leading cause of death, saw suicides go up 13 percent in 2014, increasing to 372 deaths from 328 a year earlier. (Most jails in the United States reported no deaths, according to the Justice Department.)

Nationwide, the number of deaths due to suicides went up 3.9 percent among the country’s overall population in 2014, according to the Centers for Disease Control and Prevention.

Experts say that jails tend to exacerbate suicidal behavior. In a 2010 report examining such deaths, the National Institute of Corrections said that inmates can wind up overwhelmed by the sudden stress of confinement in a jail cell.

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“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,” the authors wrote.

The same study noted that prison suicide received far less attention because there were so many more suicides in local jails, even though prison suicide was still happening at a greater rate than in the overall population.

This study also noted that most people who committed suicide in prison were “convicted of personal crimes” and “housed in single cells,” two factors seen in Hernandez’s conviction and death.

Hernandez, 27, was convicted of first-degree murder in 2015 for killing Odin Lloyd, a semiprofessional football player who was dating the sister of Hernandez’s fiance. Hernandez was being held at the Souza-Baranowski Correctional Center, a maximum-security state prison 40 miles northwest of Boston, and at the time of his death Hernandez was living in a single cell in a general population section of the prison, according to the Massachusetts Department of Corrections. (He was not being held in isolated solitary confinement, which research has shown can have catastrophic psychological effects).

The former NFL tight end was acquitted last week of two counts of first-degree murder in a separate case, which returned him to the national spotlight. As Hernandez’s original murder conviction remained under appeal at the time of his death, it could wind up being vacated because he died before that appeal process was complete.

Research has found prison inmates to be “at high risk of suicide” around the world. In a study published last year examining suicidal behavior in prisons, researchers found that mental health problems were associated with many near-lethal suicide attempts among prisoners, as were “high levels of self-reported aggression, impulsivity, hostility, childhood trauma, and hopelessness.”

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Another study of prison suicides in 2006 found that more than half of inmates who committed suicide were between the ages of 25 and 34. The same study determined that “impulsivity can be a factor in young prisoners,” adding that “upper socioeconomic status” increased the risk of suicide in prison.

The Massachusetts prison suicide rate has been significantly higher than the national average, according to the Justice Department’s statistics, while the state’s prisons have also recently seen a wave of corrections officials commit suicide.

For years, the Massachusetts prison system has drawn scrutiny for suicides, with a Boston Globe investigation in 2007 finding that most of the deaths followed poor decisions made by corrections officials. Another Globe investigation last year identified a host of problems with the treatment inmates received for mental health and substance abuse issues. In a 2011 report, a consultant hired by the state — who had previously consulted with the Corrections Department after a wave of suicides in 2006 — found “lapses in communication” in several inmate suicides during the follow-up review.

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All told, an average of a dozen people held in local, state and federal facilities die each day in the United States. Most of these deaths occur in state prisons, which hold the bulk of the country’s incarcerated population. There were 2.1 million people in custody at the end of 2015, according to the Bureau of Justice Statistics; more than 1.5 million of them were in a prison, rather than a jail. The country’s overall correctional population is far larger because it includes more than 4.6 million people on probation or parole.