Steve Wallenberg was an agent in the Federal Bureau of Investigation who had a wife and two very young sons. He, along with other agents, partook in a large task force that tried to apprehend criminal John Barnett, a killer and robber who was active in Washington, D.C.. After the group managed to surround Barnett in an airport warehouse they had staked out and the criminal took his own accomplice hostage, Agent Wallenberg was shot to death by the criminal, as was Barnett's accomplice, even though another of the task force members (Agent Fox Mulder) had had a clear shot at Barnett. Agent Wallenberg was shot in the face, after Barnett had killed his own accomplice.
While giving his testimony at Barnett's subsequent trial, Mulder explained how Agent Wallenberg had died and continued to angrily remark on the situation, even after formally being dismissed from the witness stand. Mulder was furious at Barnett for having killed Agent Wallenberg without hesitation, conscience or humanity but eventually stepped down and returned to his seat.
Upon showing Scully a video of the warehouse shootout in 1993, Agent Reggie Purdue (who had been Mulder's ASAC at the time of Agent Wallenberg's death) acknowledged that Mulder had shot Barnett (specifically, in the shoulder and hand) but not before the criminal had killed Agent Wallenberg and his own accomplice. Agent Purdue also mentioned that Mulder had never forgiven himself for the deaths.
Mulder later regretfully told Scully that, if he had shot Barnett two seconds earlier than he actually had, Steve Wallenberg would still be alive to see his son play football, as one of Wallenberg's children was now an all-star on his football team. (TXF: "Young at Heart")