In a case the FBI believes involved the murder of an off-duty Chicago cop, the federal agency brought in an outside pathologist, it met with the Cook County medical examiner’s staff in a failed effort to reverse a suicide finding, and it interviewed more than two dozen members of the Chicago Police Department.

But the FBI didn’t interview one key police figure in the course of its extraordinary, year-and-a-half-long examination of the 2015 shooting death of Sgt. Donald Markham — Kevin Duffin, the commander overseeing the detectives who determined Markham shot himself in the head.

An FBI task force didn’t try to speak with Duffin even after learning he’d had a personal relationship with Markham’s widow, Officer Dina Markham — who reported finding her husband dead after arguing with him following a night out drinking, according to records examined by the Chicago Sun-Times and interviews.

As the FBI was investigating Donald Markham’s death, his widow was found dead in her bathtub on May 28, 2017, in what the medical examiner determined to be an accidental drowning after ingesting alcohol and pills, though she’d threatened suicide.

The day of Dina Markham’s death, Duffin told detectives on the scene that he’d gotten a personal text message from Dina Markham hours earlier. That was one of many texts they had exchanged as part of what the police department has described as a personal relationship, the Sun-Times previously has reported. Duffin told detectives they’d been exchanging text messages for about six months — about the time the FBI began investigating her husband’s death.

Duffin was the only cop involved in the death investigation of Donald Markham on the day of his death who wasn’t interviewed by the FBI task force, sources tell the Sun-Times.

Duffin has had a professional friendship with FBI Supervisory Special Agent Michael Brown, an FBI agent for nearly three decades, who heads the task force, sources say.

According to Brown’s online LinkedIn profile, he oversees FBI agents and Chicago cops assigned to the Chicago FBI Homicide Task Force, providing “ongoing assistance to state and local police departments in cold-case homicides and gang-related homicides.”

More than a year after Donald Markham died, the FBI got a tip that he hadn’t killed himself and assigned the case to Brown’s task force — made up of two federal agents and two Chicago police detectives.

Brown says FBI policy prevents him from commenting. Chicago FBI spokeswoman Janine Wheeler won’t comment. Duffin could not be reached.

At the time of Donald Markham’s death, Duffin was Area North commander, overseeing all detectives on the North Side and Northwest Side, including the Old Norwood Park neighborhood where the Markhams lived.

Duffin didn’t go to the scene of Donald Markham’s death. He left the investigation in the hands of a top aide, Lt. Denis P. Walsh, who texted Duffin at 4:35 a.m. on Sept. 2, 2015 — about 90 minutes after Dina Markham had called 911 to report her husband had been shot.

“Call me so I can give you better directiond [sic],” Walsh texted Duffin, according to police records.

An hour after Duffin got that message, Walsh and three other cops put Donald Markham’s body in a police wagon to the morgue. According to the medical examiner’s office, that agency hadn’t yet been notified of the death, which would be a violation of police policy and a county ordinance.

At Walsh’s direction, the police put the bloody mattress on which Donald Markham’s body was found in another wagon, which took it to be dumped in the alley behind the Jefferson Park police station. The mattress was hauled away by a city garbage crew within hours.

City Hall’s inspector general’s office continues to investigate the police department’s handling of the Markham investigations, which also were marked by a number of other irregularities. Among them:

• A police official at the scene of Donald Markham’s death told the Sun-Times he was surprised to see Walsh in charge of the case given that he was under internal investigation over his role in the police department’s mishandling of the investigation into the death of David Koschman at the hands of then-Mayor Richard M. Daley’s nephew Richard J. “R.J.” Vanecko. Weeks later, Walsh retired rather than face firing by the Chicago Police Board for his role in the Koschman case.

• Walsh told an evidence technician there was “no need” to test Dina Markham for gunshot residue after she reported finding her husband’s body.

• Police and firefighters who responded to Dina Markham’s 911 call said she didn’t appear to have any blood on her hands or clothing, though police reports say she discovered his body while feeling around in the dark and finding blood while trying to find her keys in his pockets. Nor was any blood visible on the floor.

• Walsh at first refused to talk with investigators in the FBI-led probe of Donald Markham’s death. He later was interviewed after being subpoenaed by a Cook County grand jury. His lawyer, James McKay, says Walsh initially wouldn’t speak because investigators showed up unannounced at his home when he was busy but that he ultimately cooperated and has nothing to hide.

• Dr. J. Scott Denton, the pathologist the FBI brought in, found it unlikely that, if it were suicide, Donald Markham would have ended up in the position in which he was said to have been found. Denton said he believed that Donald Markham’s death “is best certified as a homicide.” The FBI urged chief medical examiner Dr. Ponni Arunkumar’s office to reclassify the death as a homicide. But Arunkumar’s agency insists Donald Markham killed himself.

• Police reports say Donald Markham shot himself after arguing with his wife because she wanted to stay out drinking and he wanted to go home because he had to work in the morning. But FBI reports, recently made public by the state’s attorney’s office, offer additional details. They say the Markhams had been out with another couple, police Detective Robert Voight and his wife, and that, as the Markhams continued to argue in their driveway after getting home, Dina Markham texted Voight, saying her husband had accused her of having an affair with him.

Voight told the FBI he did have an affair with her — but that it didn’t begin until two weeks after Donald Markham died and that Dina Markham had been engaging in affairs with two other Chicago cops and a firefighter, whom he didn’t identify. Voight hasn’t responded to calls seeking comment.

At the time of her husband’s death, Dina Markham had been working in the police Bureau of Internal Affairs. Less than a year later, she was transferred to Area North, working for Duffin.

Duffin, 60, is no longer commander of Area North detectives. Last September, he was reassigned to a police job with the Office of Emergency Management and Communications, which operates the city’s 911 system. A police spokesman has said the move had nothing to do with the Markham cases.

CLICK BELOW TO READ PAST COVERAGE:

• VIDEO: Working the Story: The Donald and Dina Markham mystery, March 23, 2018

• EDITORIAL: Why our city needs to care about Donald Markham’s death, March 22, 2018

• FBI files reveal cheating allegations in mysterious Chicago cop deaths, March 22, 2018

• DOCUMENTS: Read the FBI files on the mysterious deaths of 2 Chicago cops, March 22, 2018

• No charges from state’s attorney in mysterious Chicago cop death, Jan. 28, 2018

• FBI report: Mysterious Chicago cop death ‘is best certified as a homicide,’ Jan. 12, 2017

• Medical examiner rebuffs FBI findings in mysterious Chicago cop death, Dec. 20, 2017

• New text messages emerge in mysterious Chicago cop deaths, Nov. 12, 2017

• Garbage man told: Dump bloody mattress ASAP in mysterious cop death, Oct. 8, 2017

• Chicago police threw out bloody mattress in mysterious cop death, Aug. 27, 2017

• Cop drank, drowned in bathtub amid FBI probe of cop husband’s death, July 18, 2017

• Another mystery in case of Chicago cop couple’s mysterious deaths, June 18, 2017

• Chicago cop couple’s mysterious deaths under investigation, June 1, 2017