The net is “closing in” on an Oxford University worker thought to be on the run with a respected U.S. professor following the murder of a man in Chicago.
Detectives hunting Andrew Warren, 56, and Wyndham Lathem, 42, say their search has “intensified and narrowed” as they urged the pair to surrender.
Investigators believe they know the location of the pair - or where they are headed, police said on Thursday.
Warren, a treasury assistant at Oxford’s Somerville College, and Lathem, a respected professor at Northwestern University in Illinois, are suspected of killing Trenton Cornell-Duranleau.
The 26-year-old, originally from Michigan, was found stabbed to death in a 10th floor apartment Lathem in Chicago last week.
On Wednesday, a county court judge issued first-degree murder warrants for the pair while law enforcement agencies described them as “armed and dangerous”.
Cornell-Duranleau's mother, Charlotte Cornell, told reporters that the suspects are not known to the family but declined to say anything further because the suspects are “at large and there’s an ongoing investigation”.
In a statement which was released to the media, she added: “Our Family is deeply saddened by the death of our son. It is our hope that the person or persons responsible for his death are brought to justice.
“We are asking that you allow our family to process and grieve this tragedy privately. We are asking all media outlets to not contact our family, friends or associates.
“When we have had sufficient time to mourn our child's passing, we will release a more in depth statement if we believe it is appropriate to do so.”
Chicago Police Department spokesman Frank Giancamilli, would not discuss possible motives behind the killing, but characterised the June 27 attack as “domestic in nature”.
Another spokesman, Anthony Guglielmi, said the attack was so violent it broke the blade of the knife believed to have been used to kill Cornell-Duranleau.
Detectives are trying to establish how the men knew one another. They would not confirm if they believe Warren and Lathem knew each other before the former travelled to Chicago from England.
Lathem and Cornell-Duranleau did cross paths, at least electronically: They are Facebook friends.
Briton Warren is in charge of pensions and payroll at the University of Oxford's Somerville College, though his name and photograph have been removed from the school's online directory.
Lathem, a 42-year-old associate professor at Northwestern, has been a much sought-after speaker on pneumonic and bubonic plague and has been published in top scientific journals.
Cornell-Duranleau moved to Chicago from the Grand Rapids, Michigan, area after he received his cosmetology license and worked as a hair stylist. It is unclear from his Facebook page where he was working at the time of his death.