William S. Sires, a 72-year-old convicted murderer, was walking with a cane in a maximum-security prison in Shirley when he was ambushed and killed by other inmates nearly four years ago.

Sires was dragged into a cell and beaten by a man nearly half his age before a corrections officer found him 30 minutes later, so bloody and bruised he was unrecognizable, his dentures stuffed down his throat, court records show.

Three inmates were convicted for their roles in his death. But the homicide points to vulnerabilities of older inmates in the Massachusetts prison system and in particular the hazards at the Souza-Baranowski Correctional Center built for many of the state’s most dangerous inmates.

While the number of homicides in Massachusetts prisons is small — only five since 2000 — Sires’ death is a reminder of the particular dangers lurking for the growing number of older, ailing inmates in a system not set up for the elderly.

The number of inmates age 55 and older has jumped from 1,196 in 2010 to 1,582 in January, according to state data. Inmates 55 and older, the fastest growing population of prisoners nationwide, are twice as likely to be killed in prisons across the U.S. as the general population, a 2016 federal study shows.

“You have a population that is vulnerable, that is weak,” said John Reinstein, former legal director of the local chapter of the American Civil Liberties Union. “If you are responsible for their care, the question is, what steps are you taking to ensure they are safe?”

Former inmate Darrell Jones, age 52, said older inmates at the Shirley prison stay locked in their cells, to avoid what he described as a toxic atmosphere controlled by younger, gang-involved inmates. Jones was released in December after 32 years in prison when his murder conviction was vacated. “There's fights every day,” he said.

Corrections officials declined to comment specifically on Sires’ death. Spokesman Jason Dobson said the state sees inmate safety as a top priority and pointed out that its homicide rate is lower than the national average.

“While DOC considers any such death a tragedy, Massachusetts has some of the lowest rates for homicides within prisons,” Dobson said.

A 2011 state proposal to create a separate building for older inmates — citing a national trend of separating chronically ill and disabled inmates — has been put on ice. Dobson said the state is instead committed to meeting the needs of older and sick inmates with 87 specialized beds in its hospital wing in a Jamaica Plain prison, and assisted living units and a skilled nursing facility inside prisons in Norfolk and Shirley.

Massachusetts in April approved “medical parole” legislation providing early release for qualified sick and incapacitated inmates, but it is unclear yet whether the new law will reduce the number of elderly inmates.

Three of the five men killed in Massachusetts state prisons since 2000 have been older inmates. Sires was killed at the same prison as defrocked priest John Geoghan, 68, whose strangling death in 2003 prompted widespread scrutiny into the state’s role in the murder of the elderly child rapist.

Like Geoghan, Sires was murdered in a closed cell at Souza-Baranowski on a tier with one guard, court records show. An investigation into Geoghan’s death found failures in the prison system — including questions about why Geoghan was placed in the maximum-security prison in the first place — prompting a shake-up in the leadership of the state Department of Correction.

But there has been scant attention to the role of the state in the death of Sires, who was buried in a state-owned cemetery on the grounds of the North Central Correctional Institution in Gardner.

A surveillance video of Sires’ death obtained by the New England Center for Investigative Reporting shows Sires walking laps around the common area before getting into a tussle with a younger man, stalked by other inmates and dragged into a cell while others looked on.

A guard shut the cell door remotely following a request from an unidentified inmate, court records show, leaving Sires locked in to receive a fatal beating at the hands of Michael Freeman, already serving a life sentence for the 1995 stabbing death of a disabled war veteran.

Leslie Walker, executive director of the Boston-based Prisoners’ Legal Services, said the video prompts questions about why corrections officers failed to see Sires was in danger and why the bespectacled septuagenarian was at Souza at all.

“He is an old man who was murdered on their watch,” Walker said. “Older and vulnerable people should not be in a maximum-security prison.”

Souza-Baranowski, first opened in 1998, is the state’s newest and only exclusively maximum security facility. About 1,000 inmates are housed there, most of whom are restricted to their cells 20 hours a day. Prisoners vary in age from 18 to 85, according to the DOC, with an average age of 35 years old.

Joel Thompson, a staff attorney with Prisoners’ Legal Services, said it makes no sense to house men in their 80s at Souza. “Unless you are some super-human old guy, that is crazy,” he said.

The prison has long been plagued by violence. In March, 29-year-old Brandon Bottari died five days after an altercation with several inmates. His death is still under investigation. An inmate riot in 2017 was only stopped after three hours when state police filled the unit with pepper spray.

By many accounts, Sires was disliked by guards and inmates. He was serving a life sentence without the possibility of parole for the 1973 shooting death of his mother in the town of Dalton.

State police trooper Shawn Murphy described Sires as a “racist” and a “miserable old buzzard,” in a recorded interview with one of the then-suspects included in a court file. Murphy pointed out that Sires’ death was not prompting a rush of news trucks to the prison. “The governor is not lowering the flags to half-staff,” he said.

Prosecutors alleged that Freeman — following an initial dispute with Sires on the tier — conspired with two other inmates, 27-year-old Allan Erazo and 38-year-old Chad Connors, to kill him. The video shows Erazo grabbing Sires from behind and dragging him into a cell; Connors closes the curtain and walks away.

Officer David Bolduc testified that he was stationed at the guard’s podium and closed the cell door remotely on request of an inmate without checking who it was or what he wanted. Less than two minutes later, he said he was asked to re-open it and did so. Bolduc could not be reached for comment for this story.

“They obviously should have paid closer attention to who was asking that the cell be closed,’’ said Boston attorney Dana Curhan, who is representing Connors on appeal.

Corrections officer Nicholas Poladian told jurors that generally there are two guards on the tier of some 80 inmates, but he was called to a neighboring unit for about 25 minutes due to a medical emergency, leaving Bolduc by himself. He said “there’s nobody particularly watching” the surveillance videos.

He testified that when he returned to the unit, he discovered a bloodied Sires on the floor, his cane on his chest. Freeman, he explained, was standing in the back of the cell and said, “I did this. What do you want me to do now?”

Freeman pleaded guilty to first-degree murder; Connors and Erazo were convicted of second-degree murder.

But R. Michael Cassidy, a Boston College law professor, questions whether the state’s role in Sires death has been sufficiently scrutinized. He asked how the guard on duty could hear a request to close the door but not hear Sires’ assault. He asked why guards weren’t monitoring the videos.

“Sometimes it takes a tragedy to bring things to light," he said. Like the elderly outside the prison system, he said the state should make sure older inmates are protected. “They are more frail, they are more weak and they are easy prey,” he said.