The fire left just a charred pile of smoking metal and siding where the house once stood near the small city of Arlington, about 125 miles northeast of Des Moines.
Ron Humphrey, with the State Fire Marshal, said authorities believe the bodies belonged to three adults and two children. Their remains will be sent to the state medical examiner's office for positive identification. An official cause of death for the five will not be available until autopsies are completed.
Humphrey said the autopsies will determine if any foul play was involved.
"We'll look into that, but there's no indication right now that that's the case," he said.
The property was damaged so badly it would be difficult to collect enough evidence to determine the cause of the fire, Humphrey added.
"There's not much left of it," he said. "There's just no evidence left."
The Fayette County Sheriff's Office received a call at 3:35 a.m. about a house on fire. Several fire crews responded and found the house fully engulfed in flames.
Officials first retrieved four bodies, and the fifth was found by early afternoon.
Leann Baumgartner, a neighbor whose home is not close enough to see the damaged property, said she saw fire trucks zoom past during the early morning hours.
"With it being dark, we couldn't see smoke, we couldn't see anything," she said.
Humphrey said the house was owned by an elderly man who rented part of it out to others.