Nhu Nguyen, 19, was ordered held without bail at his arraignment today in Dorchester District Court on charges he pumped a single fatal bullet into Derek Matulina's chest on the platform of the Savin Hill T stop the day before Mother's Day.

Assistant District Attorney Holly Broadbent said suspect and victim knew each other and that "there had been problems and tension between the individuals in the weeks leading up to the homicide."

Broadbent said that an argument that began at the station entrance continued to the train platform, where Matulina struck Nguyen several times in the head with a padlock wrapped in a handkerchief. A third man managed to briefly sepearate the two, but then Nguyen took a gun out of his waistband and fired one shot at Matulina, hitting him in the chest, she said. She said Matulina was able to get partly up the stairs - and called 911 himself - before collapsing. Nguyen then fled to Portland, Maine, where he was arrested a few days later.

She added a Suffolk County grand jury is now investigating the incident and that police have witness statements, MBTA surveillance video and the padlock.

Nguyen's attorney, Randy Gioia, said he will argue self defense. He said Nguyen, who grew up in Dorchester, had moved to Maine to work for his father's girlfriend and that he returned to the neighborhood that weekend to visit his mother for Mother's Day. Gioia said Matulina was joined at Savin Hill by "hooded men," who may also have been armed, and that three days after Matulina's death, three men in hoodies went up to Nguyen's mother's car while she was stopped at a red light and "threatened her life in broad daylight."