On Sept. 2, 2015, Officer Dina Markham reported finding her husband Sgt. Donald Markham dead in bed at their Northwest Side home, shot in the head, his gun in hand.

The mattress — soaked with blood and other bodily fluids — would have been among the critical pieces of evidence for investigators to determine what happened.

But the Chicago Police Department quickly removed the mattress from the Markham home in Norwood Park and threw it in the garbage behind a police station, sources told the Chicago Sun-Times.

The mattress was discarded before the Cook County medical examiner’s office completed an autopsy on Donald Markham, eventually concluding he killed himself — a ruling sources have said the FBI believes is wrong.

It’s another irregularity in a case that federal agents and the Cook County state’s attorney’s office, along with the city of Chicago’s inspector general’s office, have been investigating for months.

Federal agents suspect the sergeant was killed, the Sun-Times has reported, and investigators also are trying to determine whether the police botched the case or engaged in a coverup.

But the new investigations have been complicated by the way the police handled the case, including preserving evidence such as the mattress, which was thrown out before anyone outside the police department could examine it.

Also complicating the probes has been the death of Dina Markham. Her body was found at home, in a bathtub, on May 28, six days after Sun-Times reporters asked whether she was aware of the FBI’s investigation of her husband’s death. She said she wasn’t.

The medical examiner ruled her death an accidental drowning and said she had been drinking and had taken pills. But a police spokesman says detectives are still investigating her death.

On the night her husband died, the police didn’t test Dina Markham for gunshot residue to determine whether she had fired a gun or was nearby when it was fired, the Sun-Times has reported. Records show she told investigators she had been arguing with him hours before he died.

The police investigation of Donald Markham’s death was overseen by Lt. Denis P. Walsh, who retired a few months after the case was closed as a suicide.

In addition to the trashing of the mattress, the Sun-Times has found, through interviews and records, that:

• Donald Markham’s body was loaded into the back of a police wagon and driven to the county morgue by two Chicago cops, rather than Allied Services Group, Inc., a south suburban company the police department hired to transport thousands of bodies a year because police officers said that wasn’t part of their job.

It’s unclear why the police didn’t call Allied to transport the body, which was removed before the medical examiner’s office could decide whether to send someone to examine the scene. That office has said it wasn’t even notified of Donald Markham’s death until 5:27 a.m. Sept. 2, 2015 — when two police officers already were en route to the morgue with the body, a blood-soaked pillow and bedding. As a result, the medical examiner never sent anyone to the Markham home.

Walsh says the medical examiner seldom sends anyone to the scene of a death and that the office was notified before the body was moved.

• The city’s Independent Police Review Authority — which investigates all police-related shootings in Chicago — wasn’t notified of Donald Markham’s death until 10:30 a.m. that day. That was shortly after a pathologist began the autopsy and about seven and a half hours after Dina Markham called 911 to report her husband had been shot.

“Should we have been notified earlier than 10:30 a.m.? I would say yes,” IPRA spokeswoman Mia Sissac says. “For the most part, we get notifications pretty quickly, within an hour of the actual incident happening.” IPRA also ultimately ruled Donald Markham’s death a suicide.

• After Donald Markham’s body was removed from the house, police discovered his widow’s cell phone was missing. Someone called her number, and it rang at the Cook County morgue inside her husband’s cargo shorts. A detective went to the morgue, retrieved the phone and gave it back to Dina Markham. It’s unclear why Donald Markham had it or whether police examined it.

Walsh says he’s near-certain the mattress was at the Markham home when he left there to work another case, and he says he doesn’t know what happened to it.

“I’m 100 percent sure, or 99.9 percent sure, it was there when I left,” Walsh says.

Former police Lt. Denis P. Walsh. | Sun-Times files

He also says he was questioned by the FBI and state’s attorney office.

“I was interviewed recently, very recently,” Walsh says. “I said, ‘Ask me anything you want.’ . . . I have absolutely nothing to hide . . . I couldn’t have been more cooperative.”

“I am definitely not the target or the subject,” Walsh says regarding the investigations. “Essentially, I’m a witness.”

He would not discuss what he was asked: “If you want to know what they asked me, you need to go talk to them.”

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Neither the FBI nor the state’s attorney’s office would comment. A police spokesman declined to comment on the disposal of the mattress or other questions about Donald Markham’s death because of the ongoing investigations.

Federal agents and prosecutors wanted to interview all of the police officers who responded to the Markham home the night of Donald Markham’s death, as many as 20 officers.

“The interview opened with the FBI saying, ‘We’re investigating this case because we received evidence that tells us this isn’t a suicide,’ ” says a source who spoke on the condition of not being identified. “I remember them asking if the body had been moved on the bed because the position of the body looked odd or was inconsistent with someone who had committed suicide.”

The FBI asked officers about Walsh’s handling of the Donald Markham case, according to the source, and “everybody said, ‘I don’t know. He’s a lieutenant.’ ”

At the time he oversaw the Donald Markham investigation, Walsh had been in the news as being under investigation by Inspector General Joseph Ferguson over Walsh’s role in the reinvestigation of the May 2004 death of David Koschman after being punched by Richard J. “R.J.” Vanecko, a nephew of then-Mayor Richard M. Daley.

The police closed the Koschman case without seeking criminal charges against Vanecko. But a Sun-Times investigation led to the appointment of a special prosecutor, and Daley’s nephew pleaded guilty to involuntary manslaughter.

Dina Markham and Donald Markham. | Facebook

Three months after Donald Markham’s death, Ferguson recommended that Walsh be fired over the Koschman case. The lieutenant had taken police reports from the Koschman case home while the investigation was ongoing — reports that apparently had been missing for months. Walsh retired in February 2016 rather than fight disciplinary action.

Walsh says Donald Markham’s body was transported in “a Chicago Police Department vehicle” but says that’s “in line with Chicago Police Department general orders” allowing a patrol supervisor to make that call. He says someone else did that, though he might have been involved in “the decision-making process.”

“I don’t recall who made the actual decision,” he says.

Walsh says that, in hindsight, “the more appropriate method” might have been to transport the body “with the utmost reverence” in a Chicago Fire Department ambulance, as often is done when a Chicago cop dies.

“Maybe we should have done that,” Walsh says.

Still, he says he did nothing wrong regarding the investigation of Donald Markham’s death: “I don’t know anybody who thinks I botched this case up.

“Nobody who has anything to do with the investigation believes this is a murder,” Walsh says.

READ MORE:

• Chicago cop couple’s mysterious deaths under investigation, May 31, 2017

• Another mystery in case of Chicago cop couple’s mysterious deaths, June 16, 2017