Two otters died at the Smithsonian’s National Zoo after likely ingesting a dye used by wildlife professionals to identify them, according to zoo officials.

The young adult Asian small-clawed otters, a male and a female, died late Thursday. They were part of a family of 13, seven of whom had patches of their fur dyed earlier in the day for identification purposes, a statement from the zoo said.

Zoo keepers said they later observed the two otters moving around with discomfort, and immediately took them to the Zoo’s hospital, where they died. Preliminary tests show they had a condition that affects oxygen transported by the blood, which caused their livers and kidneys to fail, according to the statement.

Officials said the most likely cause of death was the ingestion of the dye, which the Zoo has since stopped using. The dye was cleaned off of the other otters, who appear to be fine.

The Zoo has used the dye for more than 30 years without a problem.