Baltimore State's Attorney Patricia C. Jessamy said John Pontolillo will not be charged in the death of career criminal Donald D. Rice.
A review of the evidence and witness interviews "supports the finding that John Pontolillo reasonably believed he was in danger of death or serious bodily injury and was justified in striking Donald Rice," Jessamy wrote.
Pontolillo, a 21-year-old junior chemistry major from New Jersey, killed Rice by creating a huge gash in Rice's chest and nearly severing his left hand. He died at the scene.
Rice, 49, had been arrested more than two dozen times for burglary, breaking and entering and auto theft. He was released from jail just two days before he was killed last September.
Pontolillo has not spoken publicly about the case and did not immediately return phone and e-mail messages seeking comment.
On the night of the slaying, two laptops and a video game system were stolen from the home Pontolillo shared with three fellow students. According to police and prosecutors, Pontolillo went outside after police left to check on his car, which was parked in a rear garage, and grabbed a samurai sword that he kept in his bedroom.
Pontolillo spotted Rice hiding under the back porch of his home, authorities said. Pontolillo raised the sword and yelled for Rice not to move, but Rice approached him with his arms raised.
Pontolillo could not determine whether Rice was armed and struck him.
The bloody episode shocked the Hopkins community, but some students defended Pontolillo's actions and said the slaying illustrated the tense relationship between the elite university and its surrounding neighborhoods.
A Web site supporting Pontolillo offered T-shirts for sale that read, "I Am a Samurai Too." University officials, meanwhile, urged students to call 911 rather than trying to confront intruders.
Like most states, self-defense in Maryland is defined by common law rather than by statute, and prosecutors consider whether to file charges in such incidents on a case-by-case basis.
Rice's sister, Peggy Rice, told a television station in the wake of the attack that Pontolillo should be charged. She could not be located for comment Thursday.
