(Portland, OR)  --  Portland police are investigating an attack on the Burnside Bridge, after finding two "naked and bloodied" men on the structure.  Authorities say the attack happened during a robbery shortly after two o'clock this morning.  Investigators say one of the victims was cut with a box cutter after the suspects demanded all their property, including their clothing.  KOIN-TV reports two people have been arrested, and the Burnside Bridge remains open for the morning commute.