The 55-year-old man who attacked the top U.S. envoy to South Korea earlier this year has been indicted for assaulting prison staff, prosecutors said Thursday.

The Seoul Central District Prosecutors' Office said Kim Ki-jong, who was found guilty of attacking Amb. Mark Lippert with a knife at a function in Seoul, is suspected of hitting a prison officer and a surgeon when he was denied the right to go to a hospital for his ankle injury.

The staff rejected the request as the injury was treatable in jail, prosecutors said.

The Seoul District Court sentenced the 55-year-old to 12 years in prison after being convicted of attempted murder and other charges.

The attack left Lippert with deep gashes that required more than 80 stitches. (Yonhap)