The fatal stabbing of a hairstylist in Chicago was part of a sexual fantasy hatched in an online chatroom between a US professor and an Oxford University employee, a court has heard.

Their plan included killing someone and then themselves, prosecutors told a Cook County judge at a bond hearing for the men.

An Illinois prosecutor shared disturbing new details about the July 27 killing of Trenton James Cornell-Duranleau, the 26-year-old boyfriend of since-fired Northwestern University microbiology professor Wyndham Lathem.

Wyndham LathemWyndham Lathem was the boyfriend of the victim (Jim Young/AP)

The court was told the victim was stabbed 70 times at Lathem’s Chicago apartment and with such brutality that he was nearly decapitated. His throat was slit and pulmonary artery torn.

Lathem, 46, had communicated for months before with Andrew Warren, 56, about “carrying out their sexual fantasies of killing others and then themselves,” Natosha Toller, an assistant Cook County state’s Attorney, told the court.

While the prosecutor used the plural in talking about the alleged fantasy to kill, she did not say there were other victims.

Judge Adam Bourgeois deemed both men potentially dangerous and flight risks, ordering them to remain in jail pending trial on first-degree murder charges, saying: “The heinous facts speak for themselves.”

Andrew Warren arrives at a <a href=police station in Chicago (Jim YoungAP)">Andrew Warren arrives at a police station in Chicago (Jim YoungAP)

Lathem and Warren — a British citizen employed as a financial official at Oxford University — were dressed in their own clothes on Sunday at their first court appearance in Chicago.

Lathem paid for Warren’s ticket to travel to the United States and he picked Warren up at Chicago’s O’Hare International Airport a few days before the killing, the prosecutor said.

Warren spoke briefly when the judge asked if he wanted a British diplomatic office to be in contact, replying “No”. Warren, who says online he lives in Swindon, Wiltshire, has been suspended from his job as senior treasury assistant at Somerville College.

On July 26, the day before the killing, Lathem booked a room for Warren near the apartment, Ms Toller said.

Mr Cornell-Duranleau, a Michigan native, had been asleep in Lathem’s apartment when Lathem let Warren into the 10th-floor unit around 4.30am on July 27 — treading carefully so as not to wake the victim. As Warren stood in a doorway, Lathem crept up to Mr Cornell-Duranleau and began plunging a 6-inch drywall saw knife into his chest and neck, Ms Toller said.

Lathem had told Warren to take video of the killing using his mobile phone, but Warren did not end up recording it, the prosecutor said.

When Mr Cornell-Duranleau awoke, he began screaming and fought back. Lathem yelled at Warren, asking him to help subdue Mr Cornell-Duranleau, the prosecutor said.

Warren ran over to cover the victim’s mouth, then struck him in the head with a heavy lamp in an attempt to silence him, she said.

As Lathem continued to stab the victim, Warren left the room and returned with two kitchen knives. Warren bent over Mr Cornell-Duranleau and joined Lathem in stabbing him, the prosecutor said. At one point, the victim bit Warren’s hand as he struggled to fight off the attack.

She said the victim’s last words were to Lathem: “Wyndham, what are you doing?”

After showering, Lathem and Warren left the apartment an hour after the stabbing began, the prosecutor said.

They surrendered to California authorities on August 4 after an eight-day manhunt and were returned to Illinois several days ago.

The judge set a Tuesday hearing for the men, when another judge will be assigned to oversee the criminal case. Both would have a chance to enter pleas at a later arraignment.