A Mississippi man who became the first person to be executed in the state in nearly 10 years confessed to an additional killing before his execution last month, authorities announced Monday.

David Neal Cox, 50, admitted to killing his sister-in-law, Felicia Cox, in 2007 and revealed where he buried her body, according to District Attorney John Weddle. Cox was executed for the murder of his estranged wife, Kim Cox. He was also convicted of sexually assaulting his stepdaughter multiple times.