Erwin Timothy Lukesh was a serial killer who used a straight razor on his female victims and cut their tongues out. He would serve these tongues to his mother, the overly talkative and fearful Miriam Lukesh, in her sandwiches.
According to records obtained by the FBI during their investigation into the man, he was a patient at the State Psychiatric Hospital in Gaitersburg in 1995. He was diagnosed with a delusional anger sub-type that appeared soon after the suicide of his father. Four months later he was released to take care of his invalid mother.
Transporting John Doggett to another Universe
Somehow all his anger with no outlet gave him the unique ability to travel between two parallel worlds. In each of these worlds, everything was about the same but in one he carried out his sick fantasies while in the other he remained a normal law-abiding citizen. In the world where he was a killer, he ended up murdering Agent Monica Reyes and shooting and paralyzing Agent John Doggett with the firearm he stole from Reyes. This other world's Doggett walked through the portal into another nearly identical world that Lukesh had made and then was shot. When that Doggett entered the world, the other one was pushed out. In the end, his law abiding identity was lost as the paralyzed Doggett accused Lukesh of heinous crimes. He was forced to kill his mother to prevent her from telling the FBI about him. He then sought to kill Agent Monica Reyes again in this world but was gunned down by Assistant Director Brad Follmer.
Questions exist about Lukesh though based on what Reyes did soon after. She turned the life support of the paralyzed John Doggett off at his request and he died. When this was done, the world returned to the moment when the Doggett of this world disappeared from her apartment.
If the events never happened, then perhaps Lukesh was no longer dead by Follmer's hand and no longer a suspect because a paralyzed Doggett from another universe wasn't there to create any suspicion. Also Lukesh probably had a counterpart in the universe where he killed people so either that Lukesh was pushed out of that universe or he killed his counterpart in that universe.