Police are urging Prof Wyndham Lathem, 42, and Andrew Warren, 56, to "do the right thing and turn yourself in".
Arrests warrants were issued after Trenton Cornell-Duranleau, 25, was found dead in Prof Lathem's flat in Chicago on 27 July.
"The victim had several lacerations to the body," the police said.
Wyndham Lathem is a microbiology professor at Northwestern University in Illinois, who specialises in the bacteria that caused the bubonic plague.
Andrew Warren is a senior treasury assistant at Oxford's Somerville College.
The police warned that the suspects were believed to be "armed and dangerous", and were possibly driving a Hyundai sedan.
Chicago Police chief communications officer Anthony Guglielmi said on Twitter the search for the two men "will only intensify".
End of Twitter post  by @AJGuglielmi
Mr Cornell-Duranleau, originally from Michigan, worked as a cosmetologist in Chicago.
Oxford University issued a statement saying it had been in contact with the police in the UK about Mr Warren and was "ready to help the US investigating authorities in any way they need".
It added: "Andrew Warren's colleagues at Somerville College have now all been informed and are shocked to learn of the case.
"Whatever the circumstances, we would urge him to contact the US authorities as soon as possible, in the best interests of everyone concerned."
A Thames Valley Police spokesman said Mr Warren was reported missing on 25 July after having left the UK the day before.
A US-wide hunt has intensified for an American professor and an Oxford University employee suspected of stabbing a man to death in Chicago.