He and another defendant, Wyndham Lathem, 42, a USA academic, are suspected of murdering 26-year-old Trenton James Cornell-Duranleau on 27 July in Chicago.
Cornell-Duranleau was stabbed to death on July 27.
An employee of Britain's Oxford University accused of stabbing to death a Chicago-area man with a second suspect agreed on Friday to be transferred to IL from California to face a murder charge.
After the hearing, Warren's lawyer, Ariel Boyce-Smith, said: "Mr Warren is agreeable to being transported to Chicago".
The AP also reports the attack on Cornell-Duranleau was so brutal that the blade of the knife that investigators believe was used in the stabbing was broken.
KPIX: Why did you send a video to family and friends apologizing for your involvement in the crime, saying "it was the biggest mistake of my life"?
Calciomercato Real Madrid, rinnovo e clausola monstre per Isco: 700 milioni!
La presentazione formale dell'accordo relativa al rinnovo di Isco ci sarà nei prossimi giorni, dopo la Supercoppa di Spagna. Il 25enne era in scadenza a giugno e percepirà un ingaggio di 6 milioni di euro l'anno .
Prof Lathem was a microbiology professor at Northwestern University in IL and specialised in the bacteria that caused the bubonic plague.
Cornell-Duranleau suffered "lacerations and mutilations to his body, his upper body, but not to the point of decapitation", Chicago police spokesman Anthony Guglielmi said. "We've been looking a great deal, not only at the relationship between Mr Lathem and the victim, but also the connection between all three", he said. The pair had been on the run for eight days.
Police have said Warren, a senior treasury assistant at Oxford's Somerville College, arrived in the United States one or two days before the slaying. Lake Geneva authorities said the man making the donation didn't give his name. He's been under intense observation while in custody. His friends appeared at his Dublin court hearing on Monday, where his attorney described him as a "gentle soul".
Warren, a British national who arrived in the US days before the killing, is expected to make his initial appearance in a California court Friday.
Guglielmi has said officers found a "very gruesome" crime scene inside Lathem's apartment in the River North neighborhood, adding the victim was "savagely murdered". Lathem was an associate professor of microbiology and immunology at Northwestern, but was sacked after he was taken into custody. His funeral is being held Saturday in Lennon, Michigan.