D.C. police officer Joseph Burrough Newell, a 24-year veteran of the force, was shot and killed outside an Upper Marlboro, Md. home Monday evening, and police are still seeking his stepson in connection with the killing. Antwan Rayvon James, 27, should be considered armed and dangerous, said authorities during a press conference mid-day Tuesday. Prince George’s police said a preliminary investigation reveals the murder stemmed from a dispute over yard work. Authorities said Newell, 46, asked James to help with yard work, and he got angry and refused. ”It’s as simple, and as tragic, as that,” said Prince George’s County Assistant Police Chief Kevin Davis. The killing was captured on surveillence video, and shows Newell, 46, being shot in the back while standing on a ladder outside his home’s garage. Newell fell to the ground, and the gunman then stood over him and fired several more rounds into his body. ”It was an execution,” Davis said.