BOSTON (AP) — Prosecutors charged an 18-year-old man with murder Monday in the weekend shooting death of a city youth worker who investigators believe was an innocent bystander to a possible gang feud.

Keith Williams, who was originally arrested on a gun charge after Saturday’s incident, pleaded not guilty at his arraignment in Dorchester Municipal Court and was ordered held without bail, authorities said.

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Dawnn Jaffier, 26, died after shots were fired near a parade. A second woman was grazed by a bullet.

Jaffier worked in a city elementary school and a Boys & Girls Club. Friends and co-workers described her as dedicated to helping young people in the city.

‘‘Once again, the city of Boston has lost a young, vibrant, and promising member of the community,’’ Suffolk District Attorney Daniel Conley said at a news conference. ‘‘Dawnn Jaffier was exactly the sort of person we need more of. She had dedicated herself to making the city and the world a better place.’’

Conley said investigators do not believe Jaffier was the intended target of the shooting, calling her ‘‘a truly innocent victim.’’

Police Commissioner Williams Evans said there was an active and ongoing investigation and that detectives were trying to determine if the shooting stemmed from gang activity. He appealed to witnesses and anyone else who might have information to come forward.

A message left with an attorney representing Williams was not immediately returned.

‘‘The senseless violence in the city ... has to stop,’’ said Mayor Martin Walsh, who visited with the victim’s family on Monday.

Jaffier’s father showed Walsh a selfie his daughter took with the mayor a few weeks ago during a City Year event. While he did not recall the specific meeting, Walsh described Jaffier as an ‘‘incredibly special person.’’

While Boston has made inroads against violent crime, there are still far too man illegal guns on the street, the mayor said.