Jeffrey served two tours in Iraq with the military, and dispite leaving the service, he still worked as a soldier and sniper for hire.
Jeffrey felt the world had abandoned him and his beloved family, after suffering emotionally due to the fact that Summer, his daughter, was crippled by "some jackass on a cell phone" who couldn't stop his car in time from hitting her, and leaving her paralyzed from the neck down.
The Visitors, wanting a way to strike at the Fifth Column, approached Jeffrey with a request to track down and eliminate a Fifth Column cell in New York, and then obtain the information of further members and contacts for them to eliminate as well, all in exchange for using their technology to heal Summer. Jeffrey, however, willingly volunteered to perform the assassinations, as his views on the Fifth Column were in line with what the Visitors were preaching - that they were terrorists, and Jeffrey had a specific viewpoint dealing with them - to not negotiate. Also, Jeffrey, having seen his beloved daughter healed and healthy, with her life restored, was afraid the Fifth Column would unjustly drive the Visitors away from Earth.
Jeffrey tracked down and assaulted a Fifth Column cell: he expertly killed Juan Solano, and his team members Carl Bryant and Tom Maslon, but was only able to injure the fourth and final member, Alex Caruso, and couldn't stop him from escaping. Jeffrey then stole a hard drive from the cell's computers, which contained contact information on Fifth Column members, and then sent it to the Visitors. This led to the Visitors having six additional Fifth Column members murdered.
Jeffrey continued to track Alex, and was lured to try and kill him with a sniper rifle after intercepting a message on a Fifth Column website. This, however, was a trap by another Fifth Column team; led by Erica Evans, who sought to capture him for the information he initially stole. Assuming he was really a V, only after Kyle Hobbes, Evans and Father Jack Landry subdued Jeffrey, following his killing of Alex and his failed attempt to escape, was he found to be human.
Jeffrey was kept prisoner in Hobbes's underground bunker, and it was here, appearing hopeless and forlorn, he claimed that the reason he did what he did was because "we can't win" ("We Can't Win")
Jeffrey pretended to have been manipulated by the Visitors to do their bidding, and Jack was sympathetic towards him. Hobbes, however, was a trained killer, like Jeffrey, and instinctively recognised this as well as the fact Jeffrey was lying. Upon finally getting Jeffrey to admit the truth, Hobbes was granted permission by Evans to torture Jeffrey for what he knew.
Jeffrey is last seen in an unknown forest, made to dig his own grave by Hobbes. Hobbes assures him once he's finished, they will talk, and then "you're gonna beg me to kill you". It is implied Hobbes tortured Jeffrey to death. He hasn't been seen or mentioned again since. ("Heretic's Fork")