Story highlights Gustavito was 15 years old and had been at the zoo for 12 years

He was attacked in the early hours of Wednesday and died on Sunday

(CNN) Police in El Salvador are investigating a "cowardly and inhumane" attack at the National Zoological Park that killed a hippopotamus named Gustavito.

The 15-year-old hippo suffered "multiple blows on different parts of the body" from "blunt and sharp objects" in Wednesday's attack, the Ministry of Culture said in a statement.

Whoever did this has to pay. My heart goes out to you and your keepers; may you rest in peace. #Gustavito #ElSalvador pic.twitter.com/h4SiVwasCk — Gigi Perla (@gigiips22) February 27, 2017

The San Salvador zoo said veterinarians had been caring for the animal around the clock since discovering that he had been attacked. Despite their efforts, he died Sunday. The zoo plans to perform a necropsy to determine the exact cause of death.

Staff had noticed on Thursday that the hippo was displaying unusual behavior -- spending most of the day under water in his enclosure. He also had stopped eating.

The statement from the ministry said that during an examination veterinarians noticed he was suffering from "bruises, lacerations on the head and body, cramps and abdominal pain."

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