Allen “Big Al” Desdunes, a heroin dealer and turncoat federal informant, was unarmed when FBI Special Agent John Sablatura fired a single shot from a .223-caliber rifle into his left cheek and through his skull.

The shot came from such close range that it left a wide powder burn around the wound. A spent shell casing from the rifle was found on the back seat of his wife’s car.

Desdunes, 37, was dead behind the wheel in the rear parking lot of a Motel 6 in New Orleans East.

Those details, which have not been previously reported, are contained in a State Police report that stands as the only publicly available account of an FBI shooting in July 2013 that remains shrouded in secrecy.

The FBI has yet to release the findings of its internal review of the fatal shooting. Orleans Parish District Attorney Leon Cannizzaro’s office also has turned away or ignored public-records requests for the FBI’s report and other documents related to the shooting.

Prosecutors reviewed those documents before Cannizzaro, citing a lack of jurisdiction, rejected possible charges against the agent.

The investigation of New Orleans Police Department shootings in New Orleans falls under the purview of a federal consent decree that devotes a full 20 of its 122 pages to the subject of when an NOPD officer may use force and the intensive review process that is triggered when an officer does.

None of that layered review process applies when an FBI agent uses force in the city.

NOPD spokesman Tyler Gamble said NOPD officers “assisted early to help preserve the scene but not much more than that” in the investigation of Desdunes’ killing.

That contrasts with the probe by the NOPD’s “Force Investigation Team” into the fatal shooting of 22-year-old Eric Harris last month by Jefferson Parish deputies after a car chase into New Orleans.

“Federal law enforcement agencies and Louisiana State Police have jurisdiction throughout the state. So they take the lead on these types of investigations when they happen and involve their officers,” Gamble said. “In the case of JPSO, that’s an agency that only has jurisdiction in Jefferson Parish. So FIT, in conjunction with the FBI, is the lead on that investigation.”

Were a state trooper to shoot someone on Bourbon Street, for instance, the NOPD would leave it to the State Police to investigate, Gamble acknowledged.

Gamble said the NOPD has no written policy reflecting that practice but that the department intends to codify it in the future.

Three years ago, The New York Times reported that the FBI had deemed every one of the 150 fatal and nonfatal shootings by its personnel over an 18-year span to have been justified. That perfect score raised the eyebrows of watchdogs and experts who questioned the FBI’s tolerance for holding its agents accountable for missteps.

A month after that news report, Sablatura fired one shot from a Colt M4 .223-caliber rifle that struck Desdunes near his lip, split the back of his skull and lodged under the skin, the State Police report says.

A coroner’s report on the shooting, released Friday by Orleans Parish Coroner Jeffrey Rouse’s office, describes the trajectory of the bullet that pierced Desdunes’ skull as “front to back, left to right and upward.”

It describes the range of the shooting only as “close,” based on soot and stippling — skin abrasions caused by close-range gunfire — on Desdunes’ cheek.

Desdunes was wearing a gray metal wedding ring and had tattoos, the report said. One on his forearm read “Ms. Slank.” Another “extensive” tattoo on his back read in part: “Live by the gun die by the gun.”

A toxicology test found marijuana, but no other drugs, in his system.

Both the State Police report and the coroner’s report raise more questions than they answer. Neither mentions where Sablatura was positioned when he fired or what threat prompted him to shoot an unarmed man at close range.

Indeed, the State Police report doesn’t even identify Sablatura or any of the other FBI agents who apparently drove up swiftly and rammed Desdunes, who sat in the driver’s seat of his wife’s 2003 Nissan Murano, accompanied by an associate, Terry Lane, in the passenger seat. Lane has since pleaded guilty to state drug charges from a related heroin sale and is serving a 10-year prison sentence.

State Police arrived more than an hour after the shooting, and Trooper Joseph Patout wrote that he “was not provided the names of the FBI agents involved in the incident, so these names were not included in this report.”

Also gone when State Police showed up was the weapon that Sablatura fired, “which I was told was a Colt M4 .223 rifle,” Patout wrote.

The trooper was in good company in the dark: The FBI kept Sablatura’s identity under wraps for more than two years until a judge unsealed it last year as part of a civil lawsuit filed by Desdunes’ loved ones.

More details are likely to emerge at a trial slated for April in the federal civil rights case, which names several FBI agents and NOPD officers, along with the city, as defendants.

Desdunes, 37, had agreed months before his death to cooperate with the bureau in its investigation of a local heroin ring, after he was stopped in New Orleans with 100 grams of heroin, according to a police report. But he broke off contact with federal authorities after about a week, a search warrant states.

A few weeks later, an unnamed informant gave the FBI Desdunes’ new phone number. Days later, agents staked him out and watched as Desdunes disposed of “heroin-laced packaging material” in the parking lot of the motel on the Interstate 10 Service Road, the records show.

At the time of the shooting, the FBI had been tracking Desdunes’ movements for weeks, monitoring GPS data from his cellphone. The court-approved surveillance led investigators to suspect that Desdunes, after failing to touch base with his FBI contact, was peddling drugs out of the motel and perhaps stashing heroin at a separate residence.

The FBI, joined by New Orleans police, confronted him at the motel on July 30, 2013. Sablatura fired the fatal shot at about 2 p.m. that day.

State Police detectives were called to the scene and arrived about 3:20 p.m., the report says. They found the Murano’s front and rear driver’s side windows shattered, a driver’s side airbag deployed, and Desdunes behind the wheel. Small baggies containing what appeared to be heroin remained in the grip of his right hand.

Desdunes was unarmed, the report says.

An FBI-assigned Dodge Charger had front-end damage from the incident. State Police downloaded the last five seconds before impact from the Charger’s crash data recorder, the report says.

“In the five seconds recorded, the Charger was traveling 24 mph when the brakes were slightly applied, slowing the vehicle to 13 mph,” the report states. “Just before the end of the five seconds, there was a heavy application of the accelerator and increasing RPMs in the engine with no braking. It is likely that the end of the five seconds is an impact with another vehicle.”

When State Police arrived, the Murano’s engine was still running and the gearshift was in reverse. A different FBI vehicle, a Chevrolet Cruze, was pinned against its damaged rear bumper.

Desdunes was pronounced dead in the driver’s seat before State Police showed up. He was still wearing his seat belt.

Follow John Simerman on Twitter, @johnsimerman.