A 21-day-old red panda cub that was born at the National Zoo has died, the AP reported Thursday.

The cause of death was not immediately known, but zoo spokeswoman Pamela Baker-Masson says heat did not kill the cub, and veterinarians determined it was not dehydrated.

Red pandas are an endangered species. The cub was the first born to red pandas Shama and Tate, and was the first red panda born at the National Zoo in 15 years.