Complex sentence: During the 1890s, Jenson collected all the records he could find concerning the Mountain Meadows Massacre, including his own field notes, excerpts of witnesses'diaries, sworn affidavits, newspaper reports, and the transcriptions from the Mormon church's internal investigations.
Split into simpler sentences: During the 1890s, Jenson collected all the records he could find concerning the Mountain Meadows Massacre. This archive including his own field notes, excerpts of witnesses'diaries, sworn affidavits, newspaper reports, and the transcriptions from the Mormon church's internal investigations.