Complex sentence: Miller was alleged to have been hired by enemies of Garrett, but this is believed to be only a rumor because Adamson was kin to him, and Miller is believed to have been in Oklahoma at the time.
Split into simpler sentences: Miller was alleged to have been hired by enemies of Garrett. But this is believed to be only a rumor because Adamson was kin to him, and Miller is believed to have been in Oklahoma at the time.