D.C. police detective Joseph Newell Jr. never saw it coming. As he stood outside his garage on a stepladder, screwing in a light bulb, his adult stepson came up from behind, police said. Antwan James fired one bullet into his stepfather’s back and then — after the detective fell — stood over him and fired several more, police said.

“It was an execution,” said Prince George’s County Assistant Police Chief Kevin Davis.

On Tuesday — hours before James, 27, turned himself in to police — authorities said the motive was as “senseless” as they come. James’s stepfather — a well-liked police officer who had spent more than 20 years on the force — had asked him for help with yardwork at their Upper Marlboro home. And James “didn’t like it.”

“It’s as simple and as tragic as that,” Davis said.

James fled after the shooting, and police spent much of Tuesday searching for him, authorities said. They also joined family members in mourning the loss of a man they described as a dedicated officer and loving father.

Antwan Rayvon James. (Prince George's County Police Department)

Newell, 46, had worked vice in the 5th Police District, done a stint on a federal law enforcement task force and, most recently, worked in 6D, one of the city’s most dangerous areas. Cmdr. George Kucik, who oversees the city’s detective squads, said Newell was “one of those guys who loved to come to work.”

“People talked to Joe,” Kucik said. “He got information.”

Newell was father to six children — ages 4 to 24 — and although they did not live with him, he made sure to spend time with each of them, said Kesha Townsend, the mother of Newell’s two youngest children.

Townsend, 40, of Clinton, said she struggled to comprehend why anyone would want to harm him.

“He doesn’t have a mean bone in his body,” she said.

Law enforcement officials said Newell was off duty — having just returned home from the grocery store — when he asked his stepson to help him fertilize the yard about 8 p.m. Monday. The two did not argue, and Newell went outside to tend to a light bulb on the garage, according to officials and police charging documents.

But Newell’s wife yelled at her son — a former D.C. firefighter — for not listening to his stepfather, according to officials and police charging documents. James stood and said, “Oh yeah? Watch this,” according to the documents.

Officer Joseph Newell Jr. (Courtesy of D.C. police)

On Green Moss Drive — a community of mini-mansions carved out of the woods off Route 301 — gunfire rang out. James had fatally shot his stepfather, police said, his crime caught on a neighbor’s surveillance camera.

Court records show that a bench warrant had been issued last month for James’s arrest after he was accused of violating his probation in a 2011 second-degree assault case.

In that case, court records show, the 51-year-old grandfather of James’s 2-year-old son accused James of punching, kicking and choking him as he walked along Indian Head Highway. The grandfather said in an application for a protective order that James had attacked him after he confronted James about “disrespect” on a phone call.

That protective order was eventually granted, and James was given probation and ordered to complete an anger-management program in the assault case. An arrest warrant was issued last month when he failed to report to his probation agent and provide proof he had completed such a program.

James had also recently left his job at the D.C. fire department, authorities said. Alex McCray, a spokesman for the D.C. Department of Human Resources, said James resigned in August 2012 after having worked for the department since September 2007.

After the shooting, police blanketed the area with officers and tracking dogs — at one point calling for a SWAT team barricade of Newell’s home because a paramedic thought he spotted movement inside. Officials feared that it might be James, but when officers entered hours later, they determined that it was more likely an oscillating fan.

On Tuesday, officials again searched the nearby woods with a helicopter and tasked investigators with tracking down associates and others who might have known where James was. James turned himself in at police headquarters shortly before 7 p.m. Tuesday.

Police said they had obtained a warrant charging him with first-degree murder.

Keith Alexander and Magda Jean-Louis contributed to this report.