Environmentalists said they have found the body of a baby vaquita marina porpoise, one of the last of its kind, washed up dead in northern Mexico.
The newborn was found with its umbilical cord still attached on a beach in the Gulf of California by US environmental group Sea Shepherd, which said it is working with Mexican authorities to determine what killed the animal.
Scientists warned in February that there are only 30 remaining vaquita, the world's smallest porpoise. They warned it faced extinction by 2022.
Mexico's environmental protection authority said the baby vaquita's corpse would be sent to a lab in San Francisco to be tested for toxic substances, viruses and bacteria.
Sea Shepherd said the most common cause of death for the vaquita is getting caught in illegal gillnets meant to catch another endangered species, a large fish called the totoaba.
Smugglers ship the totoaba's dried swim bladder to China, where it fetches tens of thousands of dollars and is eaten in soup.
"Under the stress of fighting for its life, a mother could have discharged the calf," said Sea Shepherd activist Oona Layolle in a statement.
A female vaquita gives birth to a calf approximately once every two years, the group said.