A 10-year-old boy has been charged with first-degree murder in connection with the fatal shooting of his 7-month-old niece last week in St. Louis, according to the authorities.

The boy, whose name is not being publicly released, has been placed in the custody of child welfare services and is being psychiatrically evaluated at a hospital, said Joel Currier, a spokesman for the 22nd Judicial Circuit Court in St. Louis.

If found guilty by a judge, the boy could be placed on juvenile probation or in the custody of the state’s Division of Youth Services for as long as 11 years, until he turns 21, according to Bill Prince, president of the board of directors at the Missouri Juvenile Justice Association.

Mr. Prince said he was surprised the boy had been charged with first-degree murder, and could not remember a similar case. “I cannot recall where the premeditation, first-degree element has been there,” he said.

The authorities have not provided a motive for the killing. It was not immediately clear if the boy had a lawyer.

Police officers responded Friday afternoon to a call about a shooting at a house on North Broadway in the city’s Baden neighborhood, according to a statement from the St. Louis Metropolitan Police Department. They discovered a 7-month-old girl, Kiyomi Parker, who had been shot. She was taken to a hospital, where she died from her injuries, the police said.

According to a court filing from the police, Kiyomi was shot in the head by the 10-year-old boy using a firearm owned by her father that had been stored underneath a bedroom mattress at the home. The boy “had taken the firearm out and touched it previously,” according to the filing.

Since children under 12 cannot be prosecuted as adults in Missouri, the boy’s case will be tried through the Juvenile Division of the 22nd Judicial Circuit Court, Mr. Currier said.

That court’s mission, Mr. Currier said, is to “administer justice with compassion, dignity and respect while promoting child protection, safe communities and youth competency.”

The St. Louis circuit attorney’s office has charged the girl’s father, Ca’Marion Pawnell, 19, with second-degree murder and three counts of endangering the welfare of a child, according to the police. A 7-year-old child was also present at the home during the shooting. That child’s name and relationship to the boy and to Mr. Pawnell have not been disclosed.

Mr. Pawnell has pleaded not guilty. He has not yet secured legal representation, according to Amber Raub, a spokeswoman for the attorney’s office.

Juvenile-on-juvenile gun violence in the United States is very rare. According to an analysis of data from the Gun Violence Archive by Tom Woods, a senior associate at the Annie E. Casey Foundation, of the 384,769 shooting incidents in the United States between 2016 and 2025, only 6,451 involved children as both victims and perpetrators.

In Missouri, which has the fourth-highest rate of gun violence in the country, guns are the leading cause of death among children and teenagers, according to Everytown for Gun Safety, an advocacy organization.