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.

Hammers manager Slaven Bilic said earlier this month that Payet did not want to play for the club, but added the 29-year-old would not be sold.
But, after rejecting two bids from Marseille and signing Robert Snodgrass, West Ham have decided to sell the France international.
Payet joined West Ham from Marseille for £10.7m in June 2015.
He excelled in his first season with the London club, scoring 12 goals and earning a nomination for the PFA Players' Player of the Year award.
In February 2016 he signed a new contract to tie him to the Hammers to the summer of 2021.
He then helped France reach the final of Euro 2016, where the hosts were beaten by Portugal.
However, Payet has scored just three goals for the Hammers this season.

About 1,200 people died and 771 survived when the British liner was torpedoed by a German submarine on 7 May 1915 during World War One.
One of the largest ships in the world, it was sailing from New York to its home port of Liverpool when it was sunk off the coast of Ireland.
A minute's silence will be held at 14:10 BST - the time of the attack - at a service in Liverpool.
The disaster provoked outrage especially in Britain, with rioters targeting German businesses in Merseyside, Manchester and London.
Public opinion in the US, which had been officially neutral, was said to have changed after the death of about 130 Americans, although it was another two years before the country declared its official entry into the war, in 1917.
Ian Murphy, from Merseyside Maritime Museum, said: "In 1915, Lusitania was the world's most famous ship and the jewel in Liverpool's crown.
"Her sinking sent shockwaves around the globe and was said to have influenced America's entry to the war."
About 600 people onboard the Lusitania had links with Merseyside.
The Reverend Dr Crispin Pailing, rector of Liverpool, said: "A significant number of the crew and passengers had Liverpool connections - there is hardly a street in the Vauxhall area that wasn't affected."
The ship's owner Cunard has commissioned a memorial plaque, which will be dedicated later during a service at Liverpool Parish Church. Actors Joe McGann and Roy Carruthers will read first-hand accounts from survivors.
A painting by the artist Anthony Brown, whose great-grandfather was on the Lusitania's final voyage, will be put on display for the first time.
The service will be followed by a walk of remembrance to the Lusitania's salvaged propeller, now located at the city's Pier Head.
The disaster will also be commemorated at Cobh in the Republic of Ireland, where many victims were buried after the sinking. Last weekend, a service was held in the Isle of Man from where a fishing boat rescued about 150 people.
A US-wide hunt has intensified for an American professor and an Oxford University employee suspected of stabbing a man to death in Chicago.
[NEXT_CONCEPT]
West Ham forward Dimitri Payet is set to rejoin former club Marseille after the clubs agreed a £25m fee.
[NEXT_CONCEPT]
The centenary of the sinking of the Lusitania will be commemorated later.