The Pueblo Zoo has lost one of its red pandas after 3-year-old Phi died unexpectedly on April 25.

Emergency medical efforts were unsuccessful, the zoo said late Thursday. After review, doctors determined her death was caused by cardiomyopathy, a disease that makes it hard for the heart to deliver blood to the body.

Research suggests that as many as one in three red pandas have the disease, according to the zoo. The pandas are known to mask illnesses, making it difficult to notice symptoms. Zoologists believe this ability helps them avoid looking weak to predators.

Phi was one of four red pandas at the zoo and cohabited with Tusa. Red pandas are somewhat solitary in the wild. Phi was born June 6, 2014, in Boise, Idaho.

Update 12:10 p.m. May 13, 2018: A previous story identified Phi as a male. She is female.