Warning: This post contains graphic details.

A new BBC "Panorama" documentary reveals secret tapes recorded from inside the Saudi Consulate in Istanbul on the day of outspoken Saudi journalist Jamal Khashoggi's murder.

The audio recordings reveal grim details of what happened to Khashoggi after he entered the consulate and the conversations of the hit squad sent to kill him, as described by two UN officials who heard the tapes.

His killers call him a "sacrificial animal" and joke about having "a coffee and cigar on hand" moments before his grim murder, the officials told the BBC.

The UN's official investigation, published in June, found that Khashoggi was likely drugged with sedatives and suffocated with a plastic bag.

The full documentary releases later on Monday.

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Secret tapes that document Saudi journalist Jamal Khashoggi's murder recorded his killers calling him a "sacrificed animal" and joking about having "a coffee and cigar on hand" while butchering bodies, according to a new BBC "Panorama" documentary.

Khashoggi, an outspoken journalist who wrote for The Washington Post, died at the hands of Saudi agents inside the kingdom's consulate in Istanbul, Turkey, on October 2, 2018.

The Saudi consulate had been bugged by Turkish authorities, and the tapes recorded the grim murder and the killers' conversations that took place around the planned operation, the BBC reported, citing two people who heard the recordings.

Read more: Here's everything we know about the troubling disappearance and death of Saudi journalist Jamal Khashoggi

The Saudi consular residence in Istanbul, as seen in September 2019. Panorama team/BBC Panorama

'Has the sacrificial animal arrived?'

Included in the tapes is a phone conversation which took place between Maher Abdulaziz Mutreb, the man believed to have organized the operation, and Salah Muhammed Al Tubaigy, the forensic pathologist tasked with cutting up Khashoggi's body after the murder, according to the BBC.

The conversation took place as Khashoggi was walking toward the consulate, Baroness Helena Kennedy, a British lawyer who took part in the UN investigation into the journalist's murder, told the BBC.

"You can hear them laughing," Kennedy told the BBC.

Tubaigy is then heard saying, according to Kennedy: "I often play music when I'm cutting cadavers. And sometimes I have a coffee and cigar at hand."

The two men also "speak about... when is Khashoggi to arrive, and they say: 'Has the sacrificial animal arrived?'" Kennedy told the BBC. "That's how they refer to him."

This conversation shows that Khashoggi's murder was premeditated, Kennedy told the BBC.

Read more: These are the 15 Saudis suspected in Khashoggi's disappearance and killing

Salah Muhammed A Tubaigy as seen in surveillance camera footage published by Turkish media. Sabah

'Moving from a man who's a confident person ... [to] knowing that something fatal is about to happen'

The tapes also recorded some of Khashoggi's conversations after he enters the consulate, moments before his death.

His killers can be heard greeting the journalist as he arrives at the consulate, the BBC reported, with Kennedy sensing "rising terror" in the recording.

"There was a point where you can hear Khashoggi moving from being a man who's a confident person towards a sense of fear, a sense of anxiety, rising anxiety, rising terror, and then knowing that something fatal is about to happen," Kennedy said.

British barrister Helena Kennedy, who heard the tape recordings of Khashoggi's murder. BBC Panorama

Agnès Callamard, the UN human rights office's special rapporteur, told the BBC: "I don't know whether he thinks he could be killed. He certainly thinks that they could try to abduct him."

"He's asking 'Are you going to do that to me? Are you going to give me an injection?'" she added. "The sounds that are heard after that point indicate that he's suffocated, probably with a plastic bag over his head."

A composite image of Khashoggi and Saudi Crown Prince Mohammed bin Salman. Associated Press/Virginia Mayo; Nicolas Asfouri - Pool/Getty

The official UN investigation, published in June this year, found that he was likely drugged with sedatives and suffocated with a plastic bag.

Callamard also pointed the finger at Saudi Arabia's leadership, telling the BBC: "There is no indication under international law that this crime could be qualified under any other way but as a state killing."

Shortly after Khashoggi's disappearance last year Saudi Arabia's narrative shifted several times — from claiming that Khashoggi had left the consulate, to saying that Khashoggi was accidentally killed, to saying his death was premeditated and carried out by Saudi agents.

The kingdom's crown prince, Mohammed bin Salman, has repeatedly denied any knowledge of the murder before it took place — a claim the UN said was "inconceivable."

The BBC "Panorama" documentary releases Monday night UK time.

Read more: Mohammed bin Salman says he takes 'full responsibility' for the murder of Jamal Khashoggi but denies that he ordered it