Tilikum, the killer whale involved in dragging two trainers underwater to their deaths, is in deteriorating condition at Florida's SeaWorld due to health issues, the company said on Tuesday.
The 34-year-old male orca is suffering from a bacterial infection in its lungs that so far has not responded to treatment, SeaWorld said on its website: seaworldcares.com.
In 2010, Tilikum grabbed SeaWorld trainer Dawn Brancheau in his jaws and dragged her underwater, killing her in front of horrified onlookers during a 2010 show in Florida,
Tilikum was also one of three whales involved in the 1991 death of trainer Keltie Byrne, who had fallen into the pool at Sealand in Victoria, Canada and was dragged under water by the whales.
"We are saddened to report that over the past few weeks, Tilikum's behavior has become increasingly lethargic," the company said, adding veterinarians in Orlando were focused on managing the illness to make the whale comfortable.
A spokeswoman for the theme park company said she did not know the prognosis for the orca, which was captured in 1983 around the age of two.
"He's battling this health issue and ... right now it's deteriorating so we're trying to be more transparent and making sure everyone is aware of what we're doing and understand the amount of care we putting towards this whale," spokeswoman Aimee Jeansonne Becka said in a phone interview.
Tilikum's life was detailed in the 2013 documentary "Blackfish," which made a case against keeping orcas in captivity for entertainment and damaged the theme park's image.
Taken from his pod as a two-year-old, he has been in captivity for over 30 years.
Tilikum was also involved in the death of an Orlando tourist who was found on the whale's back in 1999 after trespassing into its tank.
SeaWorld cut jobs, lost promotional deals and faced attendance drops after the documentary and protests by People for the Ethical Treatment of Animals (PETA).
The company in February reported stable attendance at its parks in San Diego, San Antonio and Orlando last year after reporting a 4.3 per cent drop in 2014.