D.C. police on Friday charged a 21-year-old man with shooting a Takoma Park teenager once in the head and killing her on a residential street in Northwest Washington.

Authorities described the suspect as an acquaintance of the victim. They said they were still trying to determine a motive and to ascertain the relationship between 18-year-old Siobhan Nicole Lee and Alexander D. Buckley.

Police would not say whether Lee had been visiting the suspect Thursday night or early Friday.

Buckley, of Northwest Washington, was charged with first-degree felony murder. Authorities said they were exploring robbery as a possible reason for the killing.

Lee, who graduated last year from Montgomery Blair High School, was found Friday at 3:24 a.m. by a patrol officer responding to a call of an unconscious person lying on a sidewalk in the 5700 block of Fifth Street NW in the Brightwood neighborhood. Police said she was pronounced dead at the scene.

Buckley was being held Friday night pending a preliminary hearing in D.C. Superior Court.

On Twitter on Friday, friends mourned Lee’s death. Her grandmother struggled to comprehend what had happened. “I don’t have a clue,” said Belinda Lee, 61, during a brief telephone interview.

Belinda Lee said her granddaughter went to work Friday at the Giant food store in the Blair Park Shopping Center in Silver Spring, where she staffed the Starbucks stand. She did not know where her granddaughter went after leaving work.

Lee said she did not want to discuss her granddaughter in any detail. “This is a big loss,” she said.

Miki Fernandez, who has lived in a corner rowhouse at 5th and Nicholson for 18 months, said he was asleep in front of the television set in his living room when he heard police outside.

He said he didn’t hear the gunshot, but saw the body when he walked onto his front porch. Lee was lying 20 feet away on an icy sidewalk between a bus stop and a trash can, according to Fernandez. She was face-up and bundled against the cold.

“This is so sad,” Fernandez said. “You hear about how the murders are down, and then one happens right in front of your house.”

Fernandez said crime in Brightwood consists mostly of thefts — he said his bicycle was taken from his fenced-in back yard — and violence is rare. “I have no idea what happened here,” Fernandez said. “And I certainly don’t want to speculate. . . . I hope they catch whoever did this.”

The manager of the Giant store where Lee worked declined to speak to a reporter. A company spokesman released a statement saying that workers “are deeply saddened” by the death and that counselors were sent to the store to aid grieving co-workers.

Magda Jean-Louis contributed to this report.