Before he was transferred to a notoriously violent prison in West Virginia, Boston gangster and FBI snitch James "Whitey" Bulger was locked up at a Florida penitentiary known as a safe haven for marked men like him.

Despite being in his late 80s and confined to a wheelchair, prison staffers said Bulger had little to worry about while living among the general population at the Coleman II penitentiary–a mix of informants, gang dropouts, and others with targets on their back.

"We have a lot of snitches there," said Joe Rojas, who worked as a GED teacher at Coleman II until this August. "But they're safe. Like that old saying, they're among thieves."

Bulger had it particularly good. "He was in a wheelchair but he had money so he paid off a couple of the younger guys," Rojas said. "They used to bring him his lunch. They used to bring him his breakfast. And they protected him."

Why Bulger was relocated to West Virginia remains an open question.

Interviews with multiple current and former federal prison staffers–wardens, investigators and medical personnel–as well as a review of several internal prison documents revealed that several aspects of Bulger's transfer stand out as unusual and perplexing.

The current and former prison staffers believe that moving Bulger to the Hazelton penitentiary may have been the result of a series of bureaucratic failures that all added up to a "death sentence," as one former prison investigator put it, or it may have been about unloading a troublesome inmate with little regard for where he ended up.

The official reason given for Bulger's transfer from Coleman was medical in nature – that the 89-year-old's health had improved enough to allow his relocation, according to prison files obtained by NBC News. Prison officials specified on his transfer paperwork that he completed treatment at a medical facility and was in good enough health to return to the general population.

But other prison records tell a different story. Bulger wasn't at a prison hospital receiving treatment before heading off to Hazelton. He was in fact locked up in solitary confinement.

Instead of staying at Coleman, where prison workers said he was safe, Bulger was inexplicably shipped off to a general population unit at Hazelton, where he was beaten to death less than 12 hours after he arrived.

Cameron Lindsay, a retired warden at three federal facilities, called the case "a shocking failure on multiple levels."

"There's absolutely no way Bulger should have been sent to Hazelton, and he sure as heck should never have been released to the compound at Hazelton," Lindsay told NBC News.

"It's difficult to imagine how and why so many people dropped the ball on this thing."

Vito Maraviglia, a retired federal prison special investigative agent, put it in even starker terms.

"Unfortunately, it looks like they gave him the death sentence," said Maraviglia, who spent 27 years monitoring high-risk inmates and evaluating security threats in arriving prisoners.

"And for people to say, I didn't know he'd get hurt there, it's an outright lie. Either they were extremely negligent or just a complete idiot and there had to be 10 idiots because a lot of people signed off on that."

The Federal Bureau of Prisons, presented with a detailed list of questions, declined comment.

"For safety and security reasons, we do not discuss specific conditions of confinement or whether an inmate has had a disciplinary history," the agency said in a statement. "We cannot comment further due to an ongoing investigation."

Moving inmates between prisons is a common practice in the federal system. The transfers are done for multiple reasons – including serious disciplinary violations, a change in health status, a sudden security risk – and signed off on by a centralized unit in Texas and the two regional offices involved in the move.

Moving an inmate like Whitey Bulger from one prison to another would not be business as usual, according to the current and former prison staffers.

Bulger, a notorious South Boston mobster who served as the inspiration for several books and Hollywood films, was among the most high-profile of federal prisoners. With his reputation as a mob turncoat and killer of women, he also would have had no shortage of enemies in a system that comprised 122 institutions and more than 150,000 inmates.

"He ratted out a lot of people," said Rojas, the prison teacher who also runs the local prison workers union near Coleman. "You cannot put that person in, not just Hazelton, but any open yard. It's a death sentence."

The transfer process would have been started by Coleman II warden Charles Lockett, according to the current and former prison staffers.

The request, after passing through layers of federal prison bureaucracy and ultimately getting the approval from the regional offices, would have generated paperwork that would eventually cross the desk of Hazelton warden Joe Coakley, the current and former prison staffers said.

Bulger's dark past was not covered up on his intake screening form. It described him as a "Boston mobster involved in numerous murders and acts of violence…High Profile Case." His severity level was marked off as "greatest."

Still, Bulger was placed into the general population in a prison considered among the system's most violent.

"The Hazelton warden and his staff were ultimately responsible, unless, of course, ordered by a higher authority, for deciding whether or not Bulger would be released to their general population," said Lindsay.

"The prudent move would have been to safeguard Bulger by isolating him in the facility's Special Housing Unit while determining how and why he was transferred to Hazelton."

Bob Hood, a former federal Bureau of Prisons chief of internal affairs and warden at the ADX Florence supermax prison in Colorado, agreed.

"Personally, I would have placed an inmate like Bulger on protective custody status on any of the federal prisons where I served as warden," Hood said. "He would always be considered a target because of his criminal background, status as informant, and age. Never, ever, to be placed in general population status."

The official code listed on Bulger's transfer papers was 332 for "medical treatment completed." The description of 332 is as follows: "Return from medical referral center to general population after treatment for medical/physical treatment."

But the classification is baffling – none of Bulger's paperwork indicates that he spent time at a treatment facility in recent months. In April, Coleman II officials did try to transfer Bulger to a prison hospital but the request was denied. All the while, Bulger remained in solitary confinement for reasons unknown, prison records show.

"They're saying it's medical, but there has to be something else behind it," said Maraviglia, the prison investigator. "Why would they transfer him to Hazelton because of a medical thing? All of a sudden all his past security concerns aren't a concern anymore? It makes absolutely no sense."

Bulger's internal records contain another detail that stands out as unusual, according to current and former federal prison workers. He was listed as a medical care level two out of four, with four representing the most serious health concerns, despite the documents indicating that he had aortic stenosis, high blood pressure, and prostate-bladder issues.

Sandy Parr, a federal prison staffer who worked on a nursing unit for 15 years, said an 89-year-old inmate with a history of heart problems would "automatically be a care level three." Parr noted that dropping an inmate's care level to a two might have served a specific purpose: making it easier for prison officials to approve his transfer to another penitentiary.

"It's not uncommon for them to lower a person's care level to get an easy transfer," said Parr, who is also the local union president at Federal Medical Center in Minnesota.

While the current and former prison staffers interviewed by NBC News don't have direct evidence, they suspect that the real reason for Bulger's transfer lies in his disciplinary history. As NBC News previously reported, Bulger was sanctioned for threatening a nursing supervisor this February, according to prisoner records.

"Your day of reckoning is coming," he allegedly told the worker on Feb. 23, according to a law enforcement source.

Since it was a verbal threat rather than a physical assault, the infraction wasn't severe enough to justify a transfer. But starting on that day, Bulger, then 88, was placed in solitary confinement and kept there for the next eight months.

"That's a huge red flag," said Maraviglia, the prison investigator. "There was no reason for him to be locked up that long."

A Boston Police booking image dated March 16, 1953 of James 'Whitey' Bulger. Boston Police Department via EPA file

Some of the current and former prison staffers said Bulger belonged, if not at Coleman II, at a federal prison hospital.

“They should have put him in a medical facility and let him live out the rest of his life in a wheelchair," said Hood, the former warden and internal affairs chief. "That didn’t happen and for the life of me, I can’t figure out why.”

If Bulger was in fact still in good health and needed to be transferred over a disciplinary problem or security risk, the obvious place to send him would have been to the federal prisons in Terre Haute or Tucson, the current and former federal prison staffers said. Like Coleman II, those facilities are also considered "safe haven" units for informants and high-profile inmates, the current and former federal prison staffers said.

Tucson would have been out of the question, however, because Bulger was placed there in early 2014. He was shipped off to Coleman II that August after being accused of engaging in an inappropriate relationship with a female psychologist, according to the Boston Globe.

"It appears they were playing hot potato with him," said Hood. "No one wanted Bulger because he's such a high-profile inmate and you know that if anything happens to him, it will get attention around the world."

Bulger arrived at the Hazelton prison on the night of Oct. 29. The geriatric mobster agreed to be placed in a general population unit, law enforcement sources told NBC News.

Bulger was wheeled into his cell just before 10 p.m. At 8:20 a.m., three hours after the cells in his unit were unlocked, he was found murdered.

Law enforcement sources said inmates assaulted him with a padlock stuffed inside a sock before placing his bloodied body in bed to make it look like he was sleeping.

For Maraviglia, it wasn't entirely surprising that Bulger would have been asked to be in general population. But the Hazelton higher-ups should have known better than to grant his wish, he said.

"He may have felt that he was too powerful to be messed with," said Maraviglia. "We see that with major organized crime figures sometimes. But you're an 89-year-old man now. Nobody's scared of you."

Maraviglia said if he were handling the case, he would have spelled out his grave concerns, as well as Bulger's wishes, in a letter to the warden and other top brass.

"I would have said he wanted to go but I don't think he should and this is why," said Maraviglia.

The FBI and the The U.S. Attorney's Office of the Northern District of West Virginia are conducting an investigation into Bulger's killing.

Among the inmates under suspicion is a former Mafia hitman with a disdain for snitches, according to law enforcement sources and the man's lawyer. But there have been no arrests.

Hood said the probe into the murder represents the easy part. Surveillance cameras likely caught the killer or killers entering Bulger's cell.

It's the investigation that will come after, led by the Justice Department inspector general's office and focused on the federal Bureau of Prison staffers involved in the transfer, that has the potential to shake up the system.

"The public will soon know which inmates murdered Bulger, but may never understand why the prison system presented the opportunity," Hood said. "The real story will be, how was the ball dropped."