To work closely with a killer whale in a marine park requires experience, intuition, athleticism, and a whole lot of dramatic flair. Few people were better at it than top SeaWorld trainer Dawn Brancheau, who, at 40, was blond, vivacious, and literally the poster girl for the marine park in Orlando, Florida, appearing on billboards around the city. She decided she wanted to work with killer whales at the age of nine, during a family trip to SeaWorld, and loved animals so much that as an adult she used to throw birthday parties for her two chocolate Labs.

This past February 24, Brancheau was working the Dine with Shamu show, featuring SeaWorld's largest killer whale, a six-ton, 22-foot male known as "Tili" (short for Tilikum). Dine with Shamu takes place in a faux-rock-lined, 1.6-million-gallon pool that has an open-air café wrapped around one side. The families snacking on the lunch buffet that Wednesday were getting an eyeful. Brancheau bounced around on the deck of the pool, wearing a black-and-white wetsuit that echoed Tilikum's coloration, as she worked him through a few of the many "behaviors" he had learned during his nearly 27 years as a marine-park denizen. The audience chuckled at the sight of one of the ocean's top predators performing like a circus animal.

The show ended around 1:30 P.M. As the audience started to file out, Brancheau fed Tilikum some herring (he eats up to 200 pounds a day), doused him a few times with a bucket (killer whales love all sorts of stimulation), and moved over to a shallow ledge built into the side of the pool. There, she lay down in a few inches of water, talking to him and stroking him, conducting what's known as a "relationship session." Tilikum floated inert in the pool alongside her, his nose almost touching her shoulder. Brancheau was smiling, her long ponytail flaring out behind her.

One level down, a group of families gathered before the huge glass windows of the underwater viewing area. A trainer shouted up that they were ready for Tilikum. That was Brancheau's signal to instruct the orca to dive down and swim directly up to the glass for a custom photo op. It's an awesome sight when six tons of Tili come gliding out of the blue. But that day, instead of waiting for his cue and behaving the way decades of daily training in captivity had conditioned him to, Tilikum did something unexpected. Jan Topoleski, 32, a trainer who was acting as a safety spotter for Brancheau, told investigators that Tilikum took Brancheau's drifting hair into his mouth. Brancheau tried to pull it free, but Tilikum yanked her into the pool. In an instant, a classic tableau of a trainer bonding with a marine mammal became a life-threatening emergency.

Topoleski hit the pool's siren. A "Signal 500" was broadcast over the SeaWorld radio net, calling for a water rescue at G pool. Staff raced to the scene. "It was scary," Dutch tourist Susanne De Wit, 33, told investigators. "He was very wild." SeaWorld staff slapped the water surface, signaling Tilikum to leave her. The whale ignored the command. Trainers hurried to drop a weighted net into the water to try and separate Tilikum from Brancheau or herd him through two adjoining pools and into a small medical pool that had a lifting floor. There he could be raised out of the water and controlled.

Eyewitness accounts and the sheriff's investigative report make it clear that Brancheau fought hard. She was a strong swimmer, a dedicated workout enthusiast who ran marathons. But she weighed just 123 pounds and was no match for a 12,000-pound killer whale. She managed to break free and swim toward the surface, but Tilikum slammed into her. She tried again. This time he grabbed her. Her water shoes came off and floated to the surface. "He started pushing her with his nose like she was a toy," said Paula Gillespie, one of the visitors at the underwater window. SeaWorld employees urgently ushered guests away. "Will she be OK?" one asked.

Tilikum kept dragging Brancheau through the water, shaking her violently. Finally—now holding Brancheau by her arm—he was guided onto the medical lift. The floor was quickly raised. Even now, Tilikum refused to give her up. Trainers were forced to pry his jaws open. When they pulled Brancheau free, part of her arm came off in his mouth. Brancheau's colleagues carried her to the pool deck and cut her wetsuit away. She had no heartbeat. The paramedics went to work, attaching a defibrillator, but it was obvious she was gone. A sheet was pulled over her body. Tilikum, who'd been involved in two marine-park deaths in the past, had killed her.

"Every safety protocol that we have failed," SeaWorld director of animal training Kelly Flaherty Clark told me a month after the incident, her voice still tight with emotion. "That's why we don't have our friend anymore, and that's why we are taking a step back."