Tom Bonnett, Sky News
A killer whale has dragged a trainer to her death during a public performance at the SeaWorld amusement park in Florida.
Dawn had 16 years of training experience.
Dawn Brancheau, 40, was giving a lecture in front of hundreds of people after a display at the Shamu Stadium in Orlando when the whale is said to have pulled her in.
"She was rubbing the killer whale's head, and (it) grabbed her and pulled her in," said Chuck Tompkins, Corporate Curator of Zoological Operations at SeaWorld Parks & Entertainment.
Media reports have said the whale grabbed her by the waist, thrashed her about and pulled her underwater in front of an audience of visitors.
Seconds after the incident, sirens went off and SeaWorld employees rushed in to evacuate the audience.
It was not immediately clear if Dawn fell or was pulled into the tank.
"We don't know what was going through the killer whale's head. It just got done doing a wonderful session with Dawn... his behaviour was great... that's the reason that she was rubbing his head," Mr Tompkins said.
"We don't have any other specifics yet. This is a terrible, terrible situation that's happened, it's a tragedy.
"Our greatest sympathy goes out to her family."
Dan Brown, president of SeaWorld Orlando, confirmed Ms Brancheau had drowned.
He said she had been one of the park's most experienced trainers, having worked there for 16 years.
The Orlando Sentinel said the whale has killed before.
Tillikum, known as 'Tilly', was blamed for the drowning of one of his trainers in 1991 while he was performing at Sealand of the Pacific in Victoria, British Columbia, the newspaper said.
He was then sold to SeaWorld in 1992 and in 1999 was involved in another death when authorities discovered the body of a naked man lying across his back.
Authorities said the man, who had either sneaked into SeaWorld after it had shut or hidden until past closing time, probably drowned after suffering hypothermia in the water.
A former SeaWorld employee told the Sentinel Tillikum is usually kept apart from the other whales and trainers are not allowed to get into the water with him because of his past.