Alex White, Professional Snitch - NYTimes.com Eugene Richards for The New York Times Kathryn Johnston was doing pretty well until the night the police showed up. Ever since her sister died, Johnston, 92, had lived alone in a rough part of Atlanta called the Bluff. A niece checked in often. One of the gifts she left was a pistol, so that her aunt might protect herself. Alex White had the initials of the drug ring Black Mafia Family tattooed on his hand. The modest house had burglar bars on the windows and doors; there had been break-ins nearby. Eight officers approached the house, and they didn't knock. The warrant police obtained, on the basis of a false affidavit, declared they didn't have to - the house where their informant had bought crack that day, the affidavit said, had surveillance cameras, and those inside could be armed. Because they couldn't kick down the security gate, two officers set upon it with a pry bar and a battering ram in the dark around 7 p.m. on Nov. 21, 2006. Burglars, Johnston probably thought, or worse - an elderly neighbor had recently been raped. No doubt she was terrified. That is why, as the cops got closer and closer, she found her gun. And why, as the door was opening, she fired one shot. It didn't hit anyone. But it provoked a hail of return fire - 39 shots, 5 or 6 of which hit her (and some of which struck other policemen). By the time the officers burst inside, Kathryn Johnston lay in a pool of blood. Waiting outside, in the back of a police van, was the small-time dealer who told the police there were drugs in the house. He did so under pressure: earlier in the day, three members of the narcotics team, working on their monthly quota of busts, rousted him from his spot in front of a store. Tell us where we can find some weight, they said, or you're going to jail. The dealer climbed into a car with them and, a few blocks away, to save his own skin, pointed out Kathryn Johnston's house - it stood out from the others on the block because it had a wheelchair ramp in front. How did the dealer feel as he watched the home invasion, heard the fusillade of shots? And, inside the house, how long did it take for the police to realize their grave error and for some of them to decide to handcuff a fatally wounded woman and plant drugs in order to cover it up? Alex White was at his mother's house several miles away that evening. She called him downstairs when she heard the news on TV, using a nickname from his childhood (it was the way he first pronounced his own name): "Alo, a bunch of police got shot. Come and see. White went in the living room and sat next to her. The reporter said the shootings had taken place on Neal Street. White knew it was in the Bluff, where he bought and sold drugs. Earlier that evening, in fact, the Atlanta narcotics police for whom he worked as a C.I., or confidential informant - a snitch - asked him to go to the Bluff and buy drugs. His car was in the shop, so he had to say no. His mother knew none of this. Upstairs at his mother's house, he had already received a call from J. R. Smith, one of the officers from the unit. Smith sounded tense. "Hey, you got to help us out with something," White told me Smith said. Smith did not respond to a request for an interview. White said sure. He tried to be helpful to the police, do what they asked - willingness was one reason he was their most trusted informant for four years running. If White could help cover for them, Smith said, there would be good money in it for him. "You made a buy today for us," Smith explained. Two $25 baggies of crack. I did? White asked. It took him a moment to register. O.K. Who did I buy it from? Dude named Sam. Smith described the imaginary seller, told how Sam had taken his money then walked White to the back of the house and handed him the drugs as Smith and a fellow officer, Arthur Tesler, watched from a car across the street. "O.K.," White said. Smith said: "933 Neal Street. I'll call you later. Now in the living room, the TV reporter was saying how a 92-year-old woman had died in the incident, and people were suggesting that the police had shot her. Two and two came together in White's mind. They did it, he suddenly knew. They messed up. They killed that old lady. Now his heart pounded as the implications became clear. And they want me to cover for them.