Police have found no motive other than the argument over yard work, Davis said.
"It's as simple and tragic as that," he said.
James had been living with his stepfather since he was fired from the fire department sometime in the past 18 months, said Davis, who had no details about what led to the firing.
Newell had been with the Metropolitan Police Department since 1989 and investigated dangerous assaults, MPD Assistant Chief Peter Newsham said. Newell was married and had two teenage daughters in addition to his stepchildren, Newsham said.
Online court records from Maryland show James was charged last month with violating his probation on a second-degree assault charge. He also received probation before judgment in an unrelated drunken-driving case, records show.