Complex sentence: At his office in the J. Edgar Hoover Building, Mulder reviews the case with Scully, believing Joel Simmons' death is a unique example of extraterrestrial cattle mutilation on a human being.
Split into simpler sentences: At his office in the J. Edgar Hoover Building, Mulder reviews the case with Scully. Mulder believes Joel Simmons' death is a unique example of extraterrestrial cattle mutilation on a human being.