Tony Anthony Mason Jr., 40, an off-duty D.C. police sergeant, was fatally shot early Saturday while sitting in a car with another person in West Baltimore, according to authorities. (Victoria Gipson Clark/Victoria Gipson Clark)

An off-duty D.C. police sergeant, a 17-year-veteran of the force, was fatally shot early Saturday outside an apartment complex in an incident authorities are investigating as possibly domestic related.

No arrests have been made, but law enforcement officials in Baltimore and in the District said detectives are looking into links to the sergeant's personal relationships. They spoke on the condition of anonymity to discuss an active inquiry. Authorities cautioned that the investigation is in its early stages and that theories about the motive could change.

Police said the sergeant, Tony Anthony Mason Jr., 40, was in a car with a woman when he was shot. The unidentified woman sustained a leg wound.

Mason had been assigned to the District's 6th Police station, in one of the city's highest crime areas. He lived in Baltimore.

Relatives could not be reached Saturday to talk about Mason. But others who knew him were stunned.

Mason was among the latest victims in a surge of killings in Baltimore this year. A few days ago, the city surpassed 300 homicides for 2017.

"I had the best years of my career right next to Tony," said D.C. Officer Victoria Gipson Clark, who partnered with Mason from 2006 to 2013. "We had a lot of fun together."

Baltimore police said Mason and the woman were sitting in a parked car when a gunman opened fire about 12:45 a.m. Police said the shot woman was taken to a hospital for treatment. Mason died at a hospital.

The shooting occurred in the 2800 block of Elgin Avenue near the Walbrook neighborhood of West Baltimore, near Mondawnin Mall. Mason was shot on a street in front of the Elgin Townhomes, residences grouped in horseshoe-like configurations with parking lots.

Three residents said the black vehicle that was fired at had stopped in the travel lane of Elgin Road, as if it had slowed to a stop rather than being parked. It was stopped across from the complex adjacent to a large grassy field.

"I heard rapid fire," said one resident who declined to identify herself out of fear. "It wasn't just one. It was pop, pop, pop. Like six shots."

She and two other residents said they saw the wounded woman run across Elgin Road, the parking lot and into one of the apartments in the complex. No one answered the door at that address Saturday morning.

Residents described the area as quiet, although they said there had been another recent shooting. The biggest problem, they said, is that the street has become a popular spot for abandoning cars. At least six parked near where the shooting occurred had no license plates, and there were bright orange tickets warning that they would be towed.

Mason lived in a two-story red-brick duplex in the far Northeast end of Baltimore, in a blue-collar neighborhood called Frankford, about six miles from where he was shot. Neighbors said they saw him and knew he was a police officer but that they did not know him well. No one was home Saturday; mail was in front of his door, and a black Chevrolet Denali was parked in front.

Multiple law enforcement officials in the District and in Baltimore said the sergeant was on desk duty after being accused of pointing a gun at a subordinate in a police station in the District.

D.C. police confirmed that the sergeant's police powers had been suspended and that he was on desk duty. It could not be determined if those issues were connected to Saturday's shooting.

Clark, his old partner, said she and Mason rode together while in the vice unit, where he taught her street "lingo," how to buy drugs and even saved her life back in 2008, when a man pulled gun on them while they were undercover.

Despite the seriousness of their jobs, they always loosened up after work. When their shift would end, she said, they'd meet up with other officers, play poker and drink. Mason was "the life of the party," and Clark, 36, said she would get a headache from laughing so much at his jokes.

Clark acknowledged that he was "in some trouble at work" but declined to elaborate and emphasized that Mason deserves "kindness."

She said recently that Mason texted Clark to apologize if he had ever hurt her. She had no idea what he was talking about but said he explained: "I'm just trying to do right. I'm trying to change some things and be a better person."

D.C. Police Sgt. Johnathan Branch had been friends with Mason for 20 years. The two met in the academy, Branch said. Mason worked hard, but Branch said he also kept the entire office laughing with jokes, like taking a bite out of someone's sandwich and putting it back where it came from.

Branch said the two last spoke on Thursday about plans to collect blankets for the homeless. Branch said Mason's passion was "not only to clean up the streets, but to help those in need."

All day Saturday, he said that people were reaching out to him, expressing their condolences and sharing funny memories, he said.

Said Branch: "I can imagine that laugh and personality, but my heart hurts because we definitely lost a true one-of-a-kind man, an officer, a sergeant. It hurts me to my heart."

ellie.silverman@washpost.com

Lynh Bui contributed to this report.