Peterson was escorted by sheriff's deputies on Oct. 11 when he went to a funeral home in a suburb of Chicago to view the body of his mother, Betty Morphey, who died at the age of 84, Chicago Sun-Times Media reported Wednesday.
Peterson was outside prison for about an hour, a source said.
The action was not related to Peterson's notoriety, said Will County Sheriff Deputy Chief Ken Kaupas. Inmates are often allowed out under escort to attend services for newly departed loved ones, he said.
"Normal security procedures" are followed in allowing an inmate briefly outside prison walls, Kaupas said. Requests must be considered reasonable as well, so taking an inmate out of state would probably not be allowed, he added.
Decisions are made on a case-by-case basis, with the warden considering factors such as the inmate's escape risk and criminal history.
Families must pay up front for the inmate's transportation and guard escorts, said Stacey Solano, spokeswoman for the Illinois Department of Corrections.
Some 100 inmates have been allowed visitations with deceased relatives so far this year, Solano said.