SeaWorld officials say Tilikum, the orca that killed a trainer at the company's Orlando park, died from bacterial pneumonia.
SeaWorld spokeswoman Aime Jeansonne Becka announced the results of a necropsy in an email Friday.
Tilikum had been receiving treatment for a persistent bacterial infection when he died Jan. 6. He was estimated to be 36 years old.
Tilikum was profiled in the documentary "Blackfish," which helped sway popular opinion against keeping killer whales in captivity at SeaWorld parks.
Criticism over keeping orcas captive grew after Tilikum grabbed trainer Dawn Brancheau in 2010 after a "Dine with Shamu" show and pulled her into the pool, killing her. Tilikum was also involved in the deaths of two other people in the 1990s.
As SeaWorld's most prolific male orca, Tilikum sired 14 calves in the nearly 25 years he was in Orlando.