TOPEKA, Kan. (AP) -- A rare gazelle has died at the Topeka Zoo after a sudden illness.

The Topeka Capital-Journal reports (http://bit.ly/ZK9Ybo ) that the zoo euthanized the 3-year-old Addra gazelle this week. The animal became sick Wednesday and was unable to stand. The gazelle's condition worsened, however, and the staff euthanized it after about five hours.

Topeka Zoo director Brendan Wiley said the cause of death hasn't been determined, but a test showed abnormalities in some of the gazelle's organs, including its liver. More tests are being conducted.

The Addra gazelle is a critically endangered species from Africa. The zoo says there are only about 500 believed to remain in the wild.

The Topeka Zoo owns two more Addra gazelles, which appear healthy.