It started with an envelope slipped from under a prison cell door.

Peter Bistrian, an orderly sweeping the solitary-confinement unit’s floor in the Philadelphia Federal Detention Center, picked it up.

A voice from inside the cell whispered instructions to deliver the message to another inmate on the row. And although he knew neither the sender nor the intended recipient, Bistrian did as he was told.

He couldn’t have known then that his simple, furtive act would entangle him with one of Philadelphia’s most notoriously violent drug lords — Kaboni Savage — and permanently upend his life.

Within days, the former Chadds Ford financial adviser, incarcerated after a 2006 fraud conviction, would find himself routinely ferrying communications between Savage and his lieutenants. He would volunteer to secretly share those messages with federal agents building a death-penalty case.

And when the high-stakes arrangement ended in calamity, its collapse would prompt a lawsuit and trial that opened last week in the same federal courthouse where Bistrian was sentenced 12 years ago.

But when he picked up that first envelope in 2006, stuffed it into his prison jumpsuit, and agreed to deliver it to Savage, Bistrian had only one question: “Who is that?”

‘Who is Kaboni Savage?’

It was only after Bistrian alerted the prison’s guards to the notes he was passing between Savage and other members of his crew — and agreed to share those messages with agents — that he learned just whom he was dealing with.

Savage, a former boxer who had risen from street-level dealing in Hunting Park to become one of the city’s major drug traffickers, already was serving a 30-year sentence and was awaiting trial in a racketeering case that had linked him to 12 murders.

Prison had done little to chasten him. He boasted to his girlfriend on recordings that later would be played at his trial that he dreamed of “killing rats.”

And it was from the same solitary-confinement unit where Bistrian first came across him that Savage committed his most notorious crime: orchestrating a 2004 firebombing in North Philadelphia that killed four children and two women related to a former confidant, Eugene Coleman, who had agreed to inform on him to the FBI.

When Coleman was temporarily released from prison to attend the funerals, a bug placed in Savage’s prison cell caught the kingpin quipping: “They should stop off and get him some barbecue sauce, … pour it on them burnt bitches."

Agents with the Bureau of Prisons Special Investigative Services warned Bistrian of the danger he faced by informing on Savage’s crew. But as a financial consultant, Bistrian had a high tolerance for risk and was never one to walk away from an opportunity.

A federal judge labeled the former University of Delaware football player the “consummate con man” at one of his first sentencings in 1996, the result of his conviction in a $1.5 million loan scam.

When he met Savage and his crew in 2006, he was awaiting sentencing for another slate of crimes — a $400,000 check-kiting scheme to buy three Mercedes-Benzes and a Porsche and a separate racket that fleeced $1.4 million from a South African steel company.

Bistrian’s penchant for gaming the system had earned him a reputation inside prison as a reliable jailhouse snitch. Hoping to better his conditions, he had helped security staff bust a cigarette-smuggling scheme and helped avert attacks on guards by informing on other inmates.

‘What’s in it for me?’

The overtures from Savage’s group came at an opportune time. He had just been transferred to the solitary-confinement unit himself as punishment for abusing the prison’s telephone policies.

Inmates there spend 23 hours a day locked in a 6-by-8-foot cell and are let out for only an hour of recreation. As the unit’s orderly — a position granted to trusted prisoners — Bistrian had slightly more freedom to assist guards in serving meals, cleaning, and other tasks.

That made him the perfect candidate to ferry communications among Savage’s crew. For the feds, it put Bistrian in an ideal position to gather intelligence for their ongoing investigation.

“I thought, ‘What’s in it for me?’ ” Bistrian testified in a Philadelphia courtroom Tuesday. “All I want is my telephone [privileges turned back] on so I can talk to my kids and family. And I want to be returned to general population.”

A deal was reached. Bistrian secured the promises from prison staff. And each time Savage or one of his lieutenants would slip an envelope under their door, Bistrian took it to prison investigators to be copied before delivering it.

But within a month, the arrangement abruptly unraveled.

‘Stuck like Chuck’

Hours after dropping off his latest note to one of Savage’s top enforcers, Steven Northington, Bistrian returned from his orderly rounds to a cacophony from Northington’s cell.

“He was screaming like hell,” Bistrian told a jury Tuesday. “That I was a snake. That I was a rat. That I was going to be effin’ killed.”

Northington revealed that the last envelope Bistrian had delivered contained a photocopy of the note instead of the original — tipping him off that their trusted courier had been sharing their communication with the guards.

Bistrian says he begged prison staff to move him back to general population and separate him from Savage and his men.

“I did everything I could. I wrote. I stopped every officer that came by day and night,” he said. “They did nothing. My counselor said I was ‘stuck like Chuck.’ ”

Instead, security staff revoked Bistrian’s orderly status and kept him confined to his cell for the standard 23 hours. But Northington, housed just a few cells away, kept up a constant barrage of threats.

Then, on June 30, 2006 — weeks after his cover had been blown — Bistrian and Northington found themselves released into the same caged area in the recreation yard, a mistake that had immediate consequences.

Northington lunged at him and took him down with a punch. Two other members of Savage’s crew pummeled him with fists and feet. When Bistrian tried to stand, Northington slammed his face into the cage’s metal wall. He lost consciousness while prison guards rushed to break up the attack.

The beating left Bistrian with a dislocated shoulder, broken fingers, cuts across his face and head, and chipped and lost teeth.

“When I woke up, I was bleeding so hard I couldn’t see,” he recalled. “But I was screaming. I was so mad. … I told [them] this was going to happen, and it did.”

Who is to blame?

Last week, more than a decade after that attack, Bistrian, now 62, watched the grainy security camera footage of that fight from the witness stand. Although the worst of the brawl took place off camera, he fell silent as he relived the moment.

Released from prison in 2009, he is now suing eight prison officials whom he blames for blowing his cover and ignoring his warnings about the danger he was in.

He says he still suffers from nerve damage, shoulder and back injuries, post-traumatic stress, and other ailments, and is seeking punitive damages and more than $3 million in medical expenses.

Lawyers for the guards have pushed back against Bistrian’s narrative. They maintain he was warned of the risks when he approached them with the idea of informing on Savage.

They also dispute that they are to blame for blowing Bistrian’s cover, saying he fabricated the story that investigators accidentally gave him a copied note to deliver to Northington.

“Peter Bistrian is a con man. He makes up stories for his own advantage,” said Syreeta Joyce Moore, an attorney for one of the guards. “It’s what got him into prison. He did it while he was in prison and he’s continued to do it after he was released from prison.”

It’s more likely, she and her defense colleagues argue, that Bistrian’s chronic ailments stem from a subsequent attack he suffered four months after his run-in with Northington — a second rec-yard assault by an inmate with a smuggled razor blade, which guards were forced to break up by using flash-bang grenades.

(Bistrian also is suing the federal government for failing to protect him in that incident in a trial set to begin as soon as his current case concludes. The individual guards are not defendants in that proceeding.)

For all the damage Bistrian says his cooperation against Savage unleashed, his risks yielded few results.

None of the notes contained much that was useful to prosecutors and were not used at Savage’s trial. The drug kingpin was sentenced to death in 2013 and is held in a supermax prison in Colorado. Northington is serving a life sentence in a federal detention center in Florida.

As for Bistrian, he testified last week that he hoped now to salvage some sort of life after his fateful decision to pick up that first note from the solitary-confinement unit’s floor.

“They put me in the same rec yard as Northington and they all knew it,” he said. “And to this day, I’m still suffering from what they did.”