American millionaire Robert Durst has been charged with murder after being recorded in a hotel bathroom saying: “What the hell did I do? Killed them all, of course.”

The 71-year-old, who could now face the death penalty if found guilty, was filming an HBO documentary on his life called The Jinx: The Life and Deaths of Robert Durst when he made the comment seemingly without realising his microphone was still on during a toilet break.

Mr Durst, whose influential family own New York landmarks including One World Trade Center, was arrested by FBI agents on Saturday as he walked into a hotel he had booked under a false name in New Orleans.

His chilling statement aired during the final minutes of the HBO series on Sunday and yesterday, Mr Durst was formally charged the first-degree murder of Susan Berman.

The recording from the hotel bathroom would probably be admissible evidence in a murder trial, legal experts said.

“There it is, you're caught,” Mr Durst was heard whispering. “What a disaster." It was unclear whether he was speaking sincerely.

Minutes before, interviewers had presented an anonymous letter in writing similar to his own that alerted police to go look for a "cadaver" at Ms Berman's address.

He denied penning the tip-off but could not distinguish it from a separate letter he admitted writing when they were enlarged.

The admissibility of the recording may be the subject of a pre-trial hearing that would "certainly be the centrepiece of the legal battle," said Robert Mintz, a former federal prosecutor now at the McCarter & English law firm.

"It turns on whether or not there was an expectation of privacy," he added.

Mr Durst was in custody in a New Orleans jail on Monday, additionally accused of illegal possession of a firearm and possessing a firearm with a controlled substance believed to be cannabis.

He has agreed to be extradited to Los Angeles County from the city but the transfer was delayed as local prosecutors considered bringing unspecified new charges against him, his lawyer said.

Ms Berman, his friend and employee, was shot in the head in her Los Angeles home two days before Christmas in 2000.

The 55-year-old was a prominent journalist, novelist and writer. Police were due to interview her over the disappearance of Mr Durst’s wife, Kathleen, who vanished in 1982 and has since been declared legally dead.

A friend from graduate school, Ms Berman served as Mr Durst's spokesperson after his first wife disappeared, the New York Times reported.

Mr Durst was questioned in both cases but never charged and after Ms Berman’s death, he moved to Galveston in Texas, where he disguised himself as a mute woman.

He was acquitted of the murder of his elderly neighbour in the town in 2001 after saying he acted in self-defence during a struggle over a gun.

Morris Black’s body had been dismembered and thrown into the sea.

Mr Durst maintains his innocence and his defence lawyer, Dick DeGuerin , said he ws “ready to go to California and to have a trial”.

The Los Angeles Police Department claimed the timing of his arrest had nothing to do with the finale of the documentary.

Officer Rosario Herrera said on Monday she could not comment on whether detectives were aware of the finale and its contents, which producers said they handed to police before it was televised.

"The arrest was made through evidence," she told Reuters, declining to say what that evidence revealed.

Mr Durst's estranged family, who are believed to be worth at least $4bn (£2.7bn), said they were "relieved and also grateful to everyone who assisted" in his arrest.

"We hope he will finally be held accountable for all he has done," his brother, Douglas, said in a statement.

The Durst family, whose name is synonymous with power and influence in New York, controls one of the city’s largest real estate empires including the Bank of America Tower.

The actor Ryan Gosling played a character inspired by Mr Durst in the 2010 film All Good Things.

It chronicled the life of a property tycoon’s wealthy son, a series of murders linked to him and his wife’s unsolved disappearance.

Additional reporting by agencies