INDIANAPOLIS (AP) - The Indianapolis Zoo is trying to determine why a lion killed its mate.

The zoo said Friday that an adult female, Zuri, attacked 10-year-old male Nyack in their outdoor yard Monday before the zoo opened for the day. The zoo says the two had been housed together for eight years and produced three cubs in 2015.

Our Zoo family is sad to announce 10-yr-old male lion Nyack has died. Earlier this week he was injured in a physical incident with female lion Zuri. We will conduct a thorough review to attempt to understand what may have led to this. pic.twitter.com/hQD2hOAaCp

— Indianapolis Zoo (@IndianapolisZoo) October 19, 2018

Nyack was on loan from the San Diego Zoo.

Staff heard an unusual amount of roaring from the yard and tried to separate the two but that Zuri held Nyack by the neck until he stopped moving. A necropsy confirmed Nyack died of suffocation from injuries to his neck.

The zoo says daily logs kept by staff previously showed no unusual aggression, injuries or wounds between the two.