Jail deaths increased 33 percent between 2001-2019, with suicide the leading cause, according to the Bureau of Justice Statistics (BJS).
Over 77 percent of the inmates who died in 2019 were still awaiting trial.
But BJS also noted that from 2000 to 2019, an average 81 percent of jail jurisdictions in the U.S. reported no deaths during the calendar year.
At the same time, the number of deaths in state and federal prisons decreased slightly, despite a sharp rise in homicides.
BJS said the number of homicides increased from 39 in 2001 to 143 in 2019, the latest year for which data was available―representing the highest figure recorded since the bureau began publishing annual reports on Mortality in Correctional Institutions (MCI) in 2001.
The figures also showed that 4,234 persons died from all causes in state and federal prisons in 2019―a 6.6 percent decrease from the 4,515 deaths in 2018.
Almost 87 percent of the 65,027 state prisoners and 89 percent of the 7,125 federal prisoners who died in custody between 2001 and 2019 died of illness.
Cancer and heart disease alone accounted for more than 53 percent of all state prison deaths from 2001-2019.
White inmates accounted for 72 percent of those deaths. Black inmates registered the lowest suicide rate over the entire 20-year MCI collection period.
In contrast, a companion report found a steady rise in the number of jail deaths since 2001. The figure reached 1,138 in 2018, and 1,200 fatalities in 2019.
Suicide was the leading single cause of death for jail inmates in 2019, BJS said, adding that almost 77 percent of the 1,200 persons who died in local jails that year were not convicted of a crime at the time of their death, and almost 40 percent had been held for 1 week or less.
Drug or alcohol intoxication accounted for 15 percent of jail deaths in 2019, up from 4 percent in 2000.
Louisiana, Alabama, Mississippi, Tennessee, and Kentucky recorded the highest rates of state prison deaths at 500 per 100,000.