A letter written by real estate heir Robert Durst to his friend and former spokeswoman Susan Berman a year before she was killed proved to be the key piece of evidence that allowed authorities to charge the 71-year-old with her murder, according to a report. 

The Associated Press, citing a law enforcement official who spoke on condition of anonymity, reported that analysts had linked Durst's letter to Berman with one that, in Durst's own words, "only the killer could have written" to point police to Berman's body.  

Durst was charged Monday in Los Angeles with first-degree murder in the shooting of Berman, who was killed execution-style in her Beverly Hills home shortly before Christmas in 2000. He could face the death penalty under special circumstances that allege he ambushed her and murdered a witness to a crime.

The Los Angeles Times reported that the makers of "The Jinx," the acclaimed HBO documentary series about Durst, were given his letter to Berman shortly before beginning interviews for the film. It was found by Berman's stepson in a box of her belongings and was turned over to investigators approximately two years ago, around the same time the Los Angeles Police Department had re-opened the case. The Times reported that the producers locked it away in a safe deposit box instead of going to authorities.

It was not immediately clear why LAPD investigators had missed the letter during their initial investigation into Berman's murder.

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In the final episode of the series, which aired on HBO Sunday night, director Andrew Jarecki confronted Durst with similarities between the letters, the second of which informed police that they would find "a cadaver" in her house. Both were written in distinctive block handwriting and both misspelled Beverly as "Beverley." Durst admitted writing the earlier letter to Berman, but denied contacting the police. However, When Jarecki presented him with enhanced images of the misspelled "Beverley" side-by-side, Durst could not differentiate between the two. 

At that point came the now-infamous moment where Durst stepped away from the interview and went to the bathroom, still wearing the live microphone that recorded what he said next.

"There it is. You're caught!" Durst whispered. "What the hell did I do? Killed them all, of course."

The law enforcement official who spoke to the AP said that the bathroom recording was not presented to prosecutors before charges were filed because detectives were still trying to determine if the recording was tampered with in any way.

The Times reported that the filmmakers, led by Jarecki, have canceled a series of planned media appearances, saying that they expect to be called as witnesses in an upcoming trial. The paper also reported that Jarecki has given differing answers when asked about the timing of the climactic interview with Durst, as well as how much time passed before the apparently unguarded bathroom monologue was discovered by an editor. 

The Associated Press contributed to this report. 

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