Two young otters died Thursday at the National Zoo after a routine examination.

The Asian small-clawed otters had patches of fur dyed for identification purposes and received contraceptive implants, according to a zoo statement. The animals appeared to act normal after the procedure, but zookeepers later noticed that a male and female were showing signs of discomfort.

“They were immediately transported to the zoo's hospital for treatment, but unfortunately both otters died,” zoo officials said in the statement.

Necropsy results indicated the otters had methemoglobinemia, which affects oxygen transport by the blood, leading to liver and kidney failure. Ingestion of the dye was the most likely cause of death, zoo officials said.

The zoo has used the dye for more than 30 years without other problems, but stopped using it as a result of the deaths.

The young adult animals were from a new family of 13 otters. Zookeepers cleared excess dye from the other otters, and all 11 were acting normal on Friday.