Richard L. Olsen, 53, allegedly confessed to setting the fire Monday night, said Sgt. Steve Hunt of the Idaho Falls Police Department. Gasoline was used an accelerant.
Olsen, of Idaho Falls, was formally charged with burglary and arson on Wednesday and was being held in lieu of a $250,000 bond.
Postal workers were barred from the building until engineers determine whether the roof and walls were damaged so much by fire that they pose a hazard.
The walls, ceiling and floor of the lobby were blackened and waterlogged, and some of the mail inside the building was damaged by water from the sprinkler system.
``We will try to get everything delivered, but there will be delays and disruptions, and people will have to be patient with us,'' said William Scouten, acting Idaho Falls postmaster. ``It's like a 100-year flood for the post office. It's as major as it gets.''
The main post office's 104 employees have been temporarily transferred to the Eagle Rock office. However, home mail delivery will be slowed because the main post office housed the city's only mail sorting center.
The 1,300 mailboxes in the lobby will be closed and an alternate pickup was being be set up at the Eagle Rock Post Office.
Until the main post office is repaired, or postal officials find a new building to house the operation, only mail that is sorted automatically in Pocatello will be delivered.
The FBI and Bureau of Alcohol, Tobacco, Firearms and Explosives collected ash and other samples to assist postal inspectors, Idaho Falls police and fire officials in investigating the arson.
Because of the extensive damage to the building, federal charges also will be filed, said Larry Dziomba, a spokesman for the U.S. Postal Inspector.
