As one of the top-ranked officers, Captain Lewis received a recorded call on Sunday night telling him of the active shooter. As he put on his uniform to head to the Mandalay Bay, he heard that two officers had been shot, and one in the neck, but not their names. (The other officer was shot in the arm and survived; additionally, an off-duty officer attending the concert was killed.)

The brothers are identical “mirror” twins — Casey, the older one by five minutes, is the more outgoing of the two, and left-handed; Branden, the more cerebral, is right-handed. As kids, their mother encouraged them to take up paintball, figuring it would offer them training they would need if they went into the military or law enforcement. They signed up for the police academy the moment they turned 21.

The brothers are often mistaken for one another, though because Casey spends much of his time lifting weights and Branden is recovering from back surgery, Casey is the beefier of the two now. He also is the only one with a bandage on his neck.

When he first heard the shots, he said, he asked his partner, Detective Tara Brosnahan: “What the hell was that?”

They walked down the street, and heard the shots again. “434,” Detective Brosnahan yelled into her police radio, using the official term for illegal gunfire. She was the first officer to alert the shooting.

Detective Clarkson started running along the perimeter of the concert area, larger than the size of two football fields. “It was all under attack,” he said.