Russell Martin Stendal, 59, a Christian missionary and a longtime resident of Colombia, was detained Wednesday at police headquarters in Bogota.
He turned himself in after learning of his arrest warrant.
Prosecutors said they had suspected Stendal of providing "electronic and communications" support to the Revolutionary Armed Forces of Colombia (FARC) and helping set up secret stations to care for wounded and sick fighters.
But a judge on Thursday ruled that the case against Stendal was not currently sufficient to warrant his detention, and ordered his release.
