Boone County prosecutor Dan Knight told KRCG 13 he is now charging Ryan Bridger with first degree murder.
Bridger pleaded not guilty Monday and was originally charged with second degree murder for the killing of 67-year-old Aletha Turner, who was the aunt of Bridger's girlfriend.
The first degree murder charge comes with a maximum penalty of life in prison without parole.
Last month, prosecutors also charged Bridger with seven additional charges including forgery, burglary and theft.
Bridger is being held on $1 million bond.
His jury trial is scheduled to begin October 29.