Authorities in Milwaukee, Wisconsin, have arrested a 31-year-old man who they claim committed cold-blooded murder before then returning to his home, filing his annual tax return, and heading to a federal building to surrender himself. An arrest report provided to Us Weekly confirms that Dominic Francis Nosacek is facing first-degree homicide charges following the fatal shooting, which unfolded the morning of Monday, February 2. Nosacek, court records confirm, has been diagnosed as having depression and psychosis. According to police, Nosacek allegedly gunned down Angelo Nelson, 50, in an apartment complex's hallway. Prior to the shooting, the two men had been arguing - about what, though, it is not yet clear. Investigators contend Nosacek returned to his home with the murder weapon and sat down to finish his tax returns. Once finished, the arrest report claims he turned himself in to authorities at a Homeland Security building on Milwaukee's Knapp Street. Nosacek allegedly told security guards at the federal building, "I just killed someone." The guard later told police that Nosacek had described Nelson as a "terrorist," according to court records reviewed by Us. Nosacek was not armed when he arrived at the Homeland Security offices and did not appear to have any blood or visible injuries. Man Told Cops His Girlfriend Asked Him to Shoot Her in Forehead While talking to police after his arrest, Nosacek told officers that "militia men are blackmailing" him. He also repeatedly said phrases such as "euthanasia mailing me" and claimed that someone had implanted a microchip in his head. Nosacek told authorities that before the murder, people had been "shaking him" and "banging around." He said he had been threatened and that the person waiting for him - which police allege was Nelson - ran down a set of stairs, at which point he shot the fleeing man. Nosacek also told authorities that Nelson did not have anything in his hands at the time of the murder and denied the two men had argued prior to the shooting. Police later found Nelson's body at the bottom of a stairwell. Police said they recovered a gun from Nosacek's residence, which he later identified as the weapon he used to kill Nelson. Nosacek now faces life in prison if convicted. WTMJ received a statement from Nosacek's mother, Cindy Jones-Nosacek, which claimed, "Our hearts are broken at the senseless death of Angelo Nelson. To take a life is to destroy [a] universe and his family will never be the same. We pray that our son will finally get the treatment for his mental illness that he so desperately needs." Minnesota Man Beat Elderly Roommate to Death for Calling His GF a 'Fatso' Local TV station CBS 58 spoke with Nelson's mother, who is devastated by the loss of her son. "You took a life that you didn't give, you took my baby," Patricia Donald told CBS 58 on Tuesday, February 3. The investigation is ongoing. Family and friends staged a vigil Wednesday, February 4, for Nelson, where he was remembered as someone who brought energy and light to every room, according to WISN. "Every room that he walked in he exuberated energy and light," said Kashay Rideout, Nelson's cousin. "He was everybody's uncle. He was everybody's big brother. And he was just the life of the party."