Robert Durst would have gotten away with the murder of his best friend if filmmakers had not unearthed the most damning piece of evidence against him, a prosecutor said Thursday in the New York real estate heir's trial.

Durst thought they would never be able to prove he wrote the so-called cadaver note directing police to the lifeless body of Susan Berman. During lengthy interviews, he denied penning the anonymous note that he said “only the killer could have written."

But the makers of “The Jinx: The Life and Deaths of Robert Durst” discovered an envelope he sent Berman a year earlier with identical handwriting and Beverly Hills being misspelled as “Beverley” on both.

It's the smoking gun, Deputy District Attorney Habib Balian said in his closing argument in Los Angeles Superior Court.

“You can ignore everything else in this case," Balian said. “If we spent one day trying this ... here’s what you’d have: You’d have a note sent by the killer that only the killer could have written that the defendant has admitted that the killer wrote. And the defendant has stipulated, ‘I wrote the cadaver note and envelope that only the killer could have written.’”

Durst said he didn't kill Berman and doesn't know who did.

He had long denied writing the anonymous cadaver note, but he conceded at trial that he sent it after finding her dead body in her Los Angeles home in December 2000. He testified that he wanted police to find her but didn't want anyone to know he had been in her house because he feared he would be suspected of killing her.

Balian scoffed at that explanation, describing it as chapter 5 in what he called Durst's playbook to get away with murder: changing his story when necessary.

Durst had acknowledged on the witness stand that even he had difficulty imagining he could have written the note without killing Berman.

“It’s very difficult to believe, to accept, that I wrote the letter and did not kill Susan Berman,” Durst testified.

Balian said it was one of the truest things Durst said amid a ton of lies.

Durst, 78, has pleaded not guilty to murder in the point-blank shooting of Berman. Prosecutors said he killed her as she planned to speak to authorities about how she provided a phony alibi for him when his wife went missing in New York in 1982.

Kathie Durst vanished without a trace nearly 40 years ago in New York. She has been declared dead, but her body's never been found and no one has been charged with a crime related to her disappearance.

Balian used the imaginary playbook to show how Durst covered up Kathie Durst's death and how this later evolved into knocking off witnesses, such as Berman and a neighbor in Texas.

The first step was to get rid of the body. Despite a lack of forensic evidence, Balian argued Durst killed his wife in their lakeside home in Westchester County on the night she was last seen and chopped it up and buried it in the Pine Barrens of New Jersey.

Durst has always maintained he dropped his wife at a train station so she could head to the city to attend medical school the following day. He has admitted he lied when he said he spoke with her that night by phone after she reached their Manhattan apartment.

He then made it appear she was still alive.

He used Berman, the daughter of a mobster and fiercely loyal to him, to impersonate his wife the next day and phone the medical school dean to say she was sick and couldn't report to her pediatric rotation.

“They embarked on a campaign to obscure the truth, to lead the investigation in a completely wrong direction,” Balian said.

The third move was to make it appear Kathie Durst, who was close to graduation and seeking a divorce, just walked away from her life. Durst told a New York Post reporter he thought his wife had a breakdown and ran off, though he implied she may have had a run-in with criminals because of her cocaine use.

While he offered a reward for information leading to her and said he was hopeful she would return, his actions said otherwise, Balian said. Days after her disappearance, he tossed out her textbooks and clothing.

“How long are you going to hold out hope?” Balian said. “You're going to hold out zero hope because you know she's never coming home. Because you killed her.”

Durst, who is ailing and seated in a wheelchair throughout the trial, testified that he didn't kill his wife and didn't kill Berman. However, he also admitted he had lied on the witness stand and said if he had killed his wife and Berman, he would lie about it.

His lawyers are scheduled to present their final argument Thursday. Defense attorney Chip Lewis said the prosecution's scripted narrative doesn't hold up under scrutiny.