A 19-year-old man died after he was thrown against a Dallas Area Rapid Transit train as it pulled away from the station Tuesday night.
Dallas police said four to five juvenile boys attacked Octavius Lanier on the platform of the MLK Station at about 6:40 p.m.
DART spokesman Mark Ball earlier said investigators do not know the motive of the assault, which was caught on surveillance video.
"We don't know why or what -- that's what we're trying to determine right now," Ball said.
Police said early Wednesday morning that one suspect is in custody.
Detectives are interviewing witnesses and developing leads in the case, police said.
Police said the boys were seen harassing Lanier before he exited the train.
They knocked him into the side of the train as it was leaving the station, police said. Lanier, who was pinned between the train and the platform, was dragged for several yards as the train sped away.
Officials earlier said the victim was thrown against the train as it pulled into the station.
Lanier was transported to Baylor Medical Center, where he died.
Officials initially said his injuries were non-life-threatening.
Dallas homicide investigators took over the investigation from DART police.
NBC 5's Amanda Guerra contributed to this report.