A 19-year-old woman who was killed in a Southeast Washington apartment early Saturday was shot in the head by her boyfriend as she held their 2-month-old infant in her lap, according to D.C. police.

The woman, Anaiona Gaston, had been engaged in a “deep conversation” with her boyfriend, Malik Fields, the victim’s mother and police said. Fields, 21, was arrested that morning and charged with second-degree murder. Police said they found a 9mm handgun in his pocket.

Fields told police that the two were joking around and that he thought the weapon was unloaded when he pulled the trigger, according to the arrest affidavit. He said Gaston first made a motion with her hand and pretended to shoot him. He then picked up his gun and fired, he said.

The shooting occurred in a 10th-floor apartment in the 2300 block of Good Hope Road SE. Fields’s grandmother also lives there and was sleeping at the time of the 3:50 a.m. shooting. A D.C. Superior Court judge ordered Fields detained until a preliminary hearing Aug. 15.

Fields’s relatives could not be reached, and his attorney did not return a call seeking comment.

Gaston’s mother, Queen Afi Gaston, 41, runs a nonprofit advocacy group for domestic-violence victims and has counseled women and men in prison and spoken at retreats and on the Internet.

Queen Afi Gaston said her daughter often accompanied her to help raise awareness. She said her daughter was in love with Fields, but that demands and mistakes on both sides strained the relationship. She said she knew of no prior physical abuse.

Gaston said her daughter had graduated from a Prince George’s County high school and wanted to be a radiologist. She expressed her sorrow on Facebook, describing her daughter — “Lil Smiley Nana” — as “the LOVE of my life.” In one note, she wrote, “She was my counselor, my besti, the best baby girl in the world . . . I’m still waiting on you to turn the key & walk in the door one more time.”

Police said in the affidavit that Anaiona Gaston was in the apartment when Fields came home early Saturday. According to the affidavit, Fields told police that he removed the clip from his handgun and ejected one round from the chamber.

According to the affidavit, Fields said he and Gaston “began to play by making gun motions at each other.” Fields told police that Gaston shaped her hand to resemble a gun, pointed at him and said “pop, pop.” Fields said that he then pointed his gun at her and said “pop, pop,” not expecting the weapon to fire. Police said Gaston was struck in the left side of her forehead as she sat cross-legged on the bed. Fields told police that he jumped up when the gun fired and shot another round into a wall.

Police said Fields’s grandmother came into the room as Fields dressed, picked up his gun and left. He was arrested on Good Hope Road about a block from the apartment complex. Gaston was pronounced dead at the scene.