As D.C. recovers from the Navy Yard shooting that left 12 dead, investigators are zeroing in on the suspected gunman, Aaron Alexis. According to officials, the 34-year-old purchased a Remington 870 shotgun and two boxes of shells on Sunday, after passing both state and federal background checks. A lawyer for SharpShooters Small Arms Range confirmed that Alexis’s background check was approved prior to the sale. Previous to Sunday’s transaction, Alexis rented a rifle at SharpShooters and practiced at their shooting range. Earlier in the day Tuesday, investigators revealed that Alexis suffered from PTSD.