Just before 3 p.m. on April 15, Bauman was waiting among the crowd for his girlfriend to cross the finish line at the Boston Marathon. A man wearing a cap, sunglasses and a black jacket over a hooded sweatshirt looked at Jeff, 27, and dropped a bag at his feet, his brother, Chris Bauman, said in an interview.

Two and a half minutes later, the bag exploded, tearing Jeff’s legs apart. A picture of him in a wheelchair, bloodied and ashen, was broadcast around the world as he was rushed to Boston Medical Center. He lost both legs below the knee.

“He woke up under so much drugs, asked for a paper and pen and wrote, ‘bag, saw the guy, looked right at me,’” Chris Bauman said yesterday in an interview. …

While still in intensive care, Jeff Bauman gave the FBI a description of the man he saw, his brother said. Bauman’s information helped investigators narrow down whom to look for in hours of video of the attack, he said.