ST. LOUIS — A St. Louis police officer accidentally shot and killed a colleague early Thursday, authorities said.

Exactly how the shooting unfolded remains unclear. It happened around 1 a.m. at an officer’s home, Police Chief John Hayden told reporters during a news conference shortly after the shooting. Two on-duty male officers were at one of their homes when an off-duty female officer stopped by, Hayden said.

The female officer, a 24-year-old military veteran, was shot in the chest. The male officers, both 29, drove her to a hospital, where she died.

Hayden declined to answer questions about the circumstances surrounding the shooting or why the officers had gathered at the home. He did say that the officers’ arrival at the home was not preceded by a call for service or help.

Police have not released the names of any of the officers. All three had been with the department about two years.

An internal police investigation is underway. The St. Louis County prosecutor’s office is also investigating, a spokeswoman for Circuit Attorney Kim Gardner said.

Mayor Lyda Krewson on Twitter called the shooting “terribly sad.”

Jeff Roorda, business manager of the St. Louis Officers Association, said in a statement that the fallen officer “served her community and her nation with dignity and courage both as a police officer and as a member of the military.”

Roorda said the union understands that the media and the public want more information.

“So do we,” Roorda said. “But for now, we wait; we wonder; and we weep.”