It has been nearly 20 years since 6-year-old JonBenét Ramsey was found dead in her family’s Colorado home.
The 1996 killing, one of the highest profile cases in recent memory, remains unsolved. The mystery is the focus of the upcoming People Magazine Investigates episode “JonBenét: The Untold Truth,” which airs Monday night at 10 p.m. ET on Investigation Discovery.
Here are five things to know about the unsolved case that continues to grip the nation.
1. JonBenét and Her Family Were the Only People Known to Be Home at the Time of Her Death
On the day after Christmas in 1996, John Ramsey discovered his only daughter dead, with duct tape over her mouth and a cord twisted around her neck in the basement of the family’s Tudor mansion in Boulder, Colorado.
Although JonBenét’s half-brother John, 20, had spent Christmas day with the family, the only people known to have been in the home that night are the little girl, Burke, then 9, and JonBenét’s parents, Patsy and John.
• For more on the case of the JonBenét Ramsey case, watch “JonBenét: The Untold Truth” on our 10-part true crime show, People Magazine Investigates, airing Monday night at 10 p.m. ET on Investigation Discovery.
Authorities later determined that JonBenét had been sexually assaulted.
2. Her Parents and Brother Were Formally Cleared of Any Involvement
In 2008, John, Patsy and Burke were formally cleared of any involvement in the child’s death.
Then-District Attorney Mary Lacy wrote a letter to John, saying that DNA evidence cleared the trio. She apologized to the family for the scrutiny the family lived under as a result of the case.
However, current District Attorney Stan Garnett has said Lacy erred in writing the letter.
“I didn’t feel the exoneration was warranted based on the state of the evidence and the complexity of the case. And I also thought it was a very unusual thing to do in a case where there had never been any charges filed,” Garnett recently told PEOPLE.
The news came two years after Patsy died of ovarian cancer.
John remarried in 2011.
3. John Mark Karr Falsely Confessed to JonBenét’s Murder
“I was with JonBenét when she died,” he said at a press conference in Bangkok at the time. “Her death was an accident.”
Karr asserted that he committed the crime, but his former wife told reporters then that she was with Karr in Alabama the entire Christmas season in 1996.
Questions arose about the man’s story. The case against Karr was eventually dropped after DNA failed to connect him to the crime.
WATCH: Three Things To Know About JonBenét Ramsey & The Latest On The Hunt For Her Murderer
4. The Former Police Chief Said the Case Was Mishandled
“It was the Christmas holiday and we were short staffed, we faced a situation, as I said earlier, that no one in the country had ever seen before or since, and there was confusion at the scene as people were arriving before we had enough personnel on the scene,” Beckner wrote online, according to the Washington Post.
He reportedly said in the AMA session that police “should have done a much better job of controlling the crime scene,” the Post reported. He added that investigators should have gotten separate statements from John and Patsy.
5. A Private Investigator in the Case Wants a Second Look
Ollie Gray was initially hired by John and Patsy in the 1996 case. Gray continues to work on the mystery and told Today in May that the local police agency was “inexperienced” in dealing with a case of its seriousness.
“There are probably three or four people that should have been investigated earlier and still need to be investigated,” he said.
Gray pointed to a man named Michael Helgoth – who committed suicide in 1997 – as a possible suspect.
• Watch the People Magazine Investigates After Show, on the new People/Entertainment Weekly Network (PEN). Go to PEOPLE.com/PEN, or download the PEN app on Apple TV, Roku Players, Amazon Fire TV, Xumo, Chromecast, iOS and Android devices
Gray said police had their eyes on Helgoth early in the investigation but set their sights elsewhere when he killed himself. Gray also said he believes a taped confession by Helgoth may be in the possession of the man’s close family.
“He was basically a hell-raiser,” Gray said of Helgoth.