Although there remains one more, albeit small, issue with that major event we examined last time, it won't make sense without beginning to examine the remaining trips, two trips to the past, one by Senator McComb and his thugs to kill Walker and the other by Walker to attempt to get evidence from Fielding to arrest McComb. The question really is, who left the future first?
It seems complicated because Walker returns from 1994 to find the T.E.C. in chaos as it is being shut down and dismantled on orders from McComb, and as quickly as he can arrange it he persuades Matuzak to send him back. Meanwhile, on learning from his spies that Walker returned to the present, McComb decides that the only way to eliminate the threat he poses is to kill him in the past, in 1994 when he was only a D.C. police officer and not a federal temporal agent, and so plans his trip from his facility in Calverton Maryland, which Yahoo!Maps makes just over sixty miles and near one and a quarter hours away from Washington. He also has to get his goons and their weapons, and brief them. Further, this, the single most significant trip to the past reported in the film, is not detected by the T.E.C. The logic of the situation strongly suggests that Walker goes first; but the story of the film makes that impossible: unless McComb goes first, there is no history in which Melissa dies.
Consider: if Walker goes first, he reaches the past, finds Fielding, discovers the pregnancy test but does not react because he already knows Melissa is pregnant (she was not killed), organizes the proof that Fielding was in the past and leaves for the future to return her to testify. Before McComb can be arrested, he leaves for the past, changing events such that by the time Walker returns to move Fielding she is already dead. He wants to catch the killer, and also, having just seen Melissa's blood test, he realizes that he is vulnerable: if they killed Fielding in the past they might also go after him. Thus he picks up his own trail at the mall and keeps an eye on himself until that night, when the killers appear and he surprises them--not as he does in the film, but effectively nonetheless. Melissa is never killed.
Thus what must happen at this point is that McComb somehow manages to leave the future before Walker. That his trip is not detected is explained if the detection equipment has already been deactivated. He sends someone to kill Fielding in the hospital, and then organizes an assault on Walker's house that night. He kills Melissa, but inadvertently fails to kill young Walker because he was wearing a bulletproof vest when the thugs shot him.
From this history flow the events we see in the film--the Walker who never knew that Melissa was pregnant, who lives alone in an apartment, where he is ambushed by thugs. This is the Walker who leaves from the future to rescue Fielding, who finds the pregnancy test and remembers that someone attacked him on this night, makes the connections, and goes to fix the piece of history that was altered that they did not detect.
That of course underscores the problem with the detectors: we have been watching not the original history but a history which someone from the future has already altered, but the detectors did not recognize that there were time travelers running amok in 1994 because they had not yet left the future, even though the detectors in theory are scanning the past.
It also brings back that little problem, for next time, concerning the scar on McComb's face.