A D.C. man accused of fatally shooting his girlfriend and wounding their 1-year-old daughter on a Metrobus on Sunday evening has killed himself, a law enforcement source told The Washington Examiner.

Authorities had tracked Javon Steven Foster to an address on Long Island, N.Y., where he had apparently shot himself to death Monday, according to the source, who has knowledge of the investigation but was not authorized to discuss the case publicly.

Foster, 27, was wanted on a second-degree murder charge in the shooting death of 20-year-old Selina Brown.

D.C. Police Commander George Kucik said the shooting was over a domestic disagreement.

"But we don't know exactly what would cause someone to do such a thing," Kucik said.

Brown was the stepdaughter of veteran D.C. Police Officer Derrick Ferguson. The family tried to get Brown to take out restraining orders on Foster,he said, but she never did.

"I would say okay let's go file a complaint, and the next day, she would be on the phone giggling with him," Ferguson told WJLA.

This is not the first domestic violence incident for Foster. In 2010, he was found guilty of simple assault after another woman accused him of beating, biting, choking and dragging her down the stairs by her hair. He sentenced to 30 days in jail.

Police served a search warrant at his home and had held a press conference seeking his whereabouts just two hours before his body was found around 6 p.m. in Nassau County, outside of New York City.

Police said at about 5:40 p.m. Sunday, Brown held her toddler in her arms and boarded the B2 Metrobus in the 1700 block of Minnesota Avenue SE. Witnesses said Foster stepped a foot into the bus and opened fire.

Multiple bullets struck Brown, and one hit the toddler, police said. Police said the bus driver, who was also injured, drove the bus about a block forward. The mother died in the bus. The daughter, Kodie, was taken to a hospital, and was expected to live.

Kucik said investigators had obtained video from the incident, but he wouldn't get into any details about its contents.

Sunday's Metro slaying comes less than a month after an 18-year-old was fatally stabbed inside the Woodley Park Metro station on Nov. 17.

It is also the third slaying on a Metrobus so far this year, according to Metro spokesman Philip Stewart, compared to two cases in 2011.

Examiner Staff Writer Kytja Weir contributed to this report.