(Portland, OR)  --  Officials at the Oregon Zoo are trying to determine what caused six monkeys to die while in quarantine at the facility.  The group of nine cotton-top tamarins arrived at the zoo on May 22nd, and six were found dead the following day.  The zoo says the three monkeys that survived appear to be in good health.  Necropsies haven't determined the cause of death, and samples are being sent to a pathologist.