SUPERIOR, Wis. -

Authorities are investigating the death of a 37-year-old man who was being held in the Douglas County Jail.

The sheriff's office says Clifton Walker died Friday.

According to a news release from the sheriff's office, Walker told other inmates he wasn't feeling well and wanted to lie down. Paramedics were called, but Walker stopped breathing and efforts to revive him failed. He was pronounced dead at the jail.

Preliminary autopsy results found Walker suffered massive internal bleeding due to a perforated ulcer that ruptured an artery in his stomach lining.

The Duluth News Tribune reported the sheriff's office and the Wisconsin Department of Corrections are investigating.