In a small Indiana city, a lone gunman in a parking garage calmly fires into a rush-hour crowd in a public plaza, committing a massacre of five apparently random victims with six shots. The shooter leaves a perfect trail behind for the police to quickly track him down. Evidence from the scene, of a shell case and a quarter bearing the same fingerprints, points clearly to James Barr, a former Army infantry sniper. He is arrested, but will only say two things to the police: "They got the wrong guy," and "Get Jack Reacher for me." Reacher, a former Army military police officer and now a drifter, is 1500 miles away, but sees the news on CNN and gets on a bus to Indiana. Reacher has no job, no home, no car, and a shrinking savings account from his past military pay. Although Reacher has a nomadic existence, what he does have is sharp moral clarity in a modern climate of moral ambiguity.
Instead of clearing Barr, Reacher wants to assist the prosecution in convicting him. Reacher is the last person Barr would want to see for good reason. When Reacher was an investigating military policeman years past, Barr had gone on a killing spree similar to the Indiana shootout, murdering four men during the Gulf War in Kuwait City. Convoluted military politics and a technicality let Barr walk free. Reacher swore he would track the sniper down if he ever tried it again. Reacher believes Barr is guilty, but Barr's sister Rosemary is convinced of her brother's innocence and entreats lawyer Helen Rodin to defend her brother. Helen's father is the district attorney who will prosecute the case. When Reacher arrives in Indiana, Barr has been beaten so badly while in prison that he cannot remember anything about the day of the murders, leaving Reacher to form his own conclusions with the available evidence. The local NBC news reporter, Ann Yanni, is also looking for more information, and Reacher is more than willing to include her in his investigation, in exchange for the use of her car and a guaranteed public expose on the Barr case. Reacher knows that 35 yards, the parking garage shooting distance to the victims, is point-blank range for a trained military sniper like Barr. Reacher also knows the shooter missed one shot on purpose, giving Reacher one shot at the truth.
Reacher drives to Kentucky to the shooting range where the sniper practiced and learns some interesting facts from Gunny Samuel Cash, the former US Marine who owns the shooting range, which make him doubt the solidity of the presumably airtight case against Barr. Cash is unwilling to reveal information or his records to Reacher, but grudgingly agrees to talk if Reacher is able to hit a paper target dead center at 300 yards with one shot. After he succeeds, Reacher is shown 32 sheets of target paper from three years' worth of Barr's practice shootings at his range, every single sheet with dead-on maximum scores.
After the visit to the shooting range, Reacher adds Cash's information to the case evidence. Helen and Rosemary sift through the clues in a riveting analysis and finally get Reacher to conclude that Barr is innocent, which means someone set up Barr as the sniper. Someone is also trying to get Reacher off the case, which formerly seemed a slam-dunk, but is now falling apart. Reacher is teamed with Helen, the young defense lawyer working against her D.A. father with a prosecution team that has an explosive secret of its own. Reacher gets closer to the unseen enemy pulling the strings, leading him to the real perpetrators, a Russian gang masquerading as legitimate businessmen. The gang's 80-year-old capo spent much of his life in one of the infamous Soviet gulags and is known only as the Zec (prisoner). Reacher outwits the mob guards in the Russian gang's fortress, efficiently and brutally dispatching five hoods before confronting the boss and forcing him to come clean on the conspiracy from beginning to end.

Question: Who was behind the sniper killings?
Answer: A Russian gang.
Alternative answer: Russian gang's boss