More inmates died in Florida prisons last year than in any other year on record, leaving the state scrambling to identify causes and find solutions.

The tally, 428 inmate deaths in 2017, was released late Friday by the Florida Department of Corrections and showed a 20 percent increase over previous years.

The inmates who died were, on average, younger than in previous years.

"A 20 percent spike in prison deaths is of course alarming, as is the fact that it's younger inmates that are dying, rather than people who have been in there for decades," said Howard Simon, executive director of the Florida American Civil Liberties Union. "But I don't want to jump to any conclusions. Many of these are under investigations. And there's multiple causes."

Those who died in 2017 averaged 56.3 years of age. Since 2012, the average age of death in the prison system has swung between 57.1 and 58.2 years old.

The Florida prison system has long been considered one of the most dangerous by almost any metric, including inmate-on-inmate violence, use-of-force by staff and problems with delivery of health care. But there is no easy answer as to why the number of deaths spiked so drastically from one year to the next. The Department of Corrections has begun an internal investigation. The causes of death are nearly all pending further investigation.

Department Secretary Julie Jones gave an initial statement: "As our population evolves and mirrors that of society at large, we've observed that an increase in inmate deaths aligns with the increase in inmates presenting with complex substance use disorders and behavioral health issues, as well as a rise in the elderly inmate population."

Dade Correctional Institution ranked second most deadly last year among prisons that don't serve as hospitals, with 12 deaths recorded inside its walls. The year before that, it ranked number one.

State Rep. David Richardson, D-Miami Beach, has been active around Florida prison issues for years and is looking into the recent jump. He said mental health patients at Dade — one of a handful of institutions that cater to inmates with psychological issues — could contribute to the institution's high mortality numbers. Between 2010 and 2016, the Corrections Department reported a 51 percent increase in inmates with mental illnesses.

It is the same ward that infamously saw an inmate with severe schizophrenia die under bizarre circumstances in 2012. Darren Rainey was placed in a hot shower for nearly two hours after soiling himself. He collapsed and the hot water — controlled from the outside — stripped the skin from much of his body like bark. One inmate witness said Rainey was taunted by staff as he begged for mercy, although the state attorney's office declined to file charges.

The Florida prison population is getting older and inmates are serving longer sentences, thanks in part to the elimination of parole. Unlike previous years where the average age of death tended to increase slightly, apparently mirroring that aging population, in 2017 it dropped, meaning that can't be the only reason.

The youngest to die in 2017 was a 22-year-old.

Why are younger people now dying in Florida prisons? The Department of Corrections points to an increase in overdoses on synthetic drugs like K2 as a possible contributing factor.

K2, also known as "spice," is a mash-up of industrial chemicals — in this case, whatever prisoners can get their hands on, including roach spray or gasoline — sprayed on dried plant matter and then smoked. It's often chosen by inmates over marijuana because it does not show up on routine urine tests performed in the prison. Unlike marijuana, K2 has been known to cause combativeness, delusions and psychosis, and in some cases can lead to lethal overdoses.

Inmate Gaines Hill Jr. was found unresponsive by his cellmate at Columbia Correctional Institution Annex on May 18, 2017. He was slumped over on his bunk and cold to the touch, according to the written record of the Florida Department of Law Enforcement investigation into the case. He was transferred to Shands Lake Shore Hospital in Lake City, where he was declared dead. The cause: an overdose on K2.

"Anything can get in," said Ron McAndrew, a former Florida warden who now works as a prison consultant. "K2 has become the new enemy."

Addiction and overdoses continue to rise across Florida, according to a November 2017 report by the FDLE. And prisons are not immune to issues outside their walls. Simon from the ACLU said that Florida prisons are full of addicts and drug offenders, due to Florida's mandatory sentencing laws.

Santa Rosa Correctional Institutions, known as one of the state's most violent prisons, took over as leader with 13 deaths in 2017, up from seven the year before. Other top spots went to Charlotte Correctional, Martin and Columbia, likewise considered among the state's most violent institutions. In the case of Charlotte and Columbia, the average age of death dropped several years. (Zephyrhills was also a leader but is known for housing elderly and infirm inmate populations.)

Charlotte Correctional, in particular, has been in the spotlight in recent years for a spate strange deaths. Perhaps most infamously, inmate Matthew Walker was killed by officers after they roused him in the middle of the night as part of routine cell checks that a grand jury said were clearly intended to "harass and aggravate" inmates. The grand jury declined to indict the officers for lack of sufficient evidence, however, when other officers refused to testify.

On Oct. 3, 2017, 25-year-old Brodrick Campbell, in minimum security for involvement in a burglary, died at Charlotte, making him the sixth of eight inmates to die at the institution last year. His fiancee, Dieunide Amady, said that the autopsy called Campbell's death a suicide, but she didn't believe it. She said she had seen him two days earlier, on a weekly Sunday visit she made with their four children, and his mother and sister.

"He spent a lot of time with the kids. Playing with them. Joking with them. Our visits were always fun," Amady said. She said Campbell talked constantly about coming home. "He was nowhere near suicidal at all. He was a very positive thinker." The FDLE investigation into Campbell's death is ongoing.