'Heartbreaking': 17-year-old killed at DC high school in 3rd shooting of child this week

Washington, D.C., Mayor Muriel Bowser on Wednesday said officials must "act urgently" to combat crime in the nation's capital after a 17-year-old was fatally shot in a high school parking lot while class was still in session, according to police -- the third shooting involving a child in the District since Sunday.

The teenager, who has not been publicly identified, died following a confrontation at Roosevelt High School that resulted in gunshots, D.C. Metropolitan Police said. He was found with one gunshot wound and was pronounced dead at the hospital.

Law enforcement has begun a preliminary investigation and is asking for the public's help in finding the shooter. They said the teen had been in school at Roosevelt earlier Wednesday and authorities are investigating what time he left.

Assistant Chief of Police Morgan Kane told reporters later Wednesday that police do not believe students inside the school faced any danger.

A source familiar with the situation told ABC News that that D.C. Public Schools planned to deploy immediate mental health and social workers to assist with crisis support for students and staff at Roosevelt.

"I can't overstate how heartbreaking it is, how heartbroken I am right now, for our kids with what we see happening as a police agency in the city," Kane said. "But what I will tell you is that it just makes us more deliberate and intentional in our resolve to put our hands around what is happening with our kids."

On Sunday, 10-year-old Arianna Davis was shot in the upper body while driving home with family following Mother's Day celebrations, police have said. Metropolitan Police said she was "accidentally hit in a barrage of gunfire."

On Wednesday, authorities said Davis had died.

Separately, before dawn on Monday, a 12-year-old girl was shot after a bullet went through the window of her home and struck her in the leg as she was sleeping, according to authorities. She was transported to a hospital with non-life-threatening injuries.

Speaking to reporters on Wednesday, Mayor Bowser said: "We don't want to see any gun violence in our city, certainly not among our children. We regard our schools as the safest place for our children."

Bowser told reporters that she felt there "needs to be a fundamental shift" with "how we talk about consequences and how we're working with young people that we know are in trouble."

Asked to elaborate, Bowser said: "I think it needs to be on how to get young people who are using guns and make sure that they can't hurt other people."

She pointed to new legislative efforts aimed at increasing public safety, unveiled on Monday, which she said she hopes the city council "urgently" takes up before the end of the summer. The proposal would increase penalties for some violent crime and illegal gun possession.

"We can't think of our juvenile rehabilitation system as punishment that courts and judges don't regard as a way to help keep young people out of trouble," she said, adding that her administration is supporting the "idea of respite or shelter care before a young person gets himself in trouble or hurt somebody or ends up being a victim of crime."

ABC News' Arthur Jones contributed to this report.