NEW ORLEANS — Robert Durst could not explain away the similarities between his handwriting and a letter he said ‘‘only the killer could have written’’ that alerted police to his friend’s shooting 15 years ago.
Confronted with new evidence by the makers of a documentary about his life, the troubled millionaire blinked, burped oddly, pulled his ear, and briefly put his head in his hands before denying he was the killer.
Then he stepped away from the tense 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 to himself before running the tap water. ‘‘What the hell did I do? Killed them all, of course.’’
That moment did not just make for a captivating finale to a six-part documentary on the eccentric life of an heir to a New York real estate fortune.
It also may have given police and prosecutors the evidence they needed to close the long-cold case of a mobster’s daughter. Susan Berman was felled by a bullet to the back of her head as investigators prepared to find out what she knew about the disappearance of Durst’s wife in 1982.
Los Angeles prosecutors filed a first-degree charge against Durst on Monday that could trigger the death penalty.
In Louisiana, Durst was rebooked on charges of being a convicted felon in possession of a firearm and possession of a weapon with a controlled dangerous substance, a small amount of marijuana.
Authorities did not immediately know whether prosecutors would try to keep Durst in Louisiana on those charges before he is sent to Los Angeles.
The charges came after two years of investigation and allege he lay in wait with a gun and murdered a witness, special circumstances that could carry a death sentence if prosecutors decide to pursue it.
Durst, 71, who was arrested at a New Orleans hotel on the eve of Sunday’s final episode of HBO’s “The Jinx:The Life and Deaths of Robert Durst,’’ agreed Monday to face trial for the murder of Berman, who had vouched for him in public after his wife vanished.
The makers of the HBO show said Durst had waved off his lawyer’s advice to stay quiet before granting them two lengthy interviews. They also say he knew he was being recorded throughout and that they shared any evidence they gathered with police long before broadcasting the film.
Legal experts say the bathroom tape could become key evidence.
‘‘Any statement that the defendant makes that they want to use against him, they can use against him,’’ said Andrea Roth, a law professor at the University of California Berkeley. ‘‘Even if it’s sketchy and only in context appears to make him look guilty.’’
Kerry Lawrence, a defense attorney in Westchester County, N.Y, said Durst’s lawyers will have to try to explain away his comments, perhaps dismissing them as a joke.
‘‘Prosecutors would argue it was a candid moment of self-reflection, and he I assume will argue that he knew he was still being recorded, and this was either said in jest or he was being facetious or sarcastic or was being provocative,’’ Lawrence said. ‘‘I don’t think it’s quite the smoking gun.’’
The documentary showed filmmaker Andrew Jarecki confronting Durst with a copy of an anonymous letter that alerted Beverly Hills police to go look for a ‘‘cadaver’’ at Berman’s address.
Durst offered that whoever sent it was ‘‘taking a big risk. You’re sending a letter to police that only the killer could have written.’’
Then, in the final episode, Jarecki revealed another envelope, which Durst acknowledged mailing to Berman, that has similar writing in block letters and also misspelled the address as ‘‘Beverley.’’
‘‘I wrote this one but I did not write the cadaver one,’’ Durst said. But when shown an enlargement of both copies, Durst could not distinguish them.
Former Westchester County prosecutor Jeanine Pirro seemed stunned when the filmmakers showed her Durst’s previously unknown letter to Berman, saying ‘‘the jig is up.’’
She believes it was her reopening of the cold case into Kathleen Durst’s 1982 disappearance that provoked the murder of Berman.
Now, she said, Durst’s own words can convict him.
‘‘It was a spontaneous statement, a classical exception to the hearsay rule,’’ Pirro told Fox’s ‘‘Good Day New York.’’