preston wright , 23 , killed his girlfriend , sarah owen , 21 , with a knife .
wright then turned the weapon on himself inside the home in norman .
officers were met by a boy , thought to be wright 's brother , at the house .
he said owen had been cleaning out the garage when she got into a fight with wright .
eyewitnesses first saw the couple arguing outside of the home .
police found the couple dead inside the house with severe stab wounds .