Complex sentence: Robert and Sienna head toward the Old City because the cylinder must have something to do with Dante, but they find that local police and Italian military police have sealed the bridges and are searching for them.
Split into simpler sentences: Robert and Sienna head toward the Old City, believing the cylinder must have something to do with Dante. However, they find that Florentini police and Carabinieri officers have sealed the bridges and are searching for them.