PORTLAND, MAINE—A 10-year-old girl was charged Thursday with manslaughter in the death of a 3-month-year-old infant whose mother said had ingested medication and been suffocated.

The girl, who is not being identified, is the youngest person to be charged with manslaughter in Maine in at least 25 years. The infant, Brooklyn Foss-Greenaway, of Clinton in central Maine, had been left overnight with an adult baby sitter in nearby Fairfield.

The sitter called police early July 8 to report that the infant was not breathing, authorities said. Emergency workers who arrived minutes later reported that child was unresponsive.

The death of any child under age 3 triggers an automatic investigation in Maine, and detectives uncovered some “troubling signs” before the state medical examiner declared the death a homicide, said Stephen McCausland, spokesman for the Maine Department of Public Safety.

The death was declared a homicide Wednesday, but investigators withheld further details on the cause of death.

Brooklyn’s mother, Nicole “Nicki” Greenaway, of Clinton, said the 10-year-old was the sitter’s daughter. Authorities told her the baby had ingested medication to treat attention-deficit disorder and been suffocated, she said, adding that she also saw bruises on her daughter’s body.

“I feel a little bit of relief that they’re charging her daughter at this point, but the mom really needs to be responsible. She’s the one I left my daughter with,” Greenaway said.

The 10-year-old girl was already in the custody of the Maine Department of Health and Human Services when a summons was delivered to her attorney on Thursday, McCausland said.

Police declined to identify the girl, who is due in juvenile court in October. Her attorney didn’t immediately return a call from The Associated Press.

Greenaway said that the 10-year-old had changed her baby’s diaper in the past, but that she had told the sitter an adult needed to be present at all times. Instead, the infant, who was reportedly fussy that night, was sleeping in a portable crib in the same room as the 10-year-old, Greenaway said.

When Greenaway finally saw her daughter at a funeral home, the infant had a black eye, bruises on the bridge of her nose and marks that looked like fingerprints on her cheeks, she said.

McCausland said he couldn’t comment on whether charges could be brought against the sitter. A person who answered the phone at a listing for her said Thursday evening that she was not available. The AP is not naming the sitter because it could make the juvenile suspect’s identity known.

The charge against the 10-year-old was filed after detectives consulted with the attorney general’s office, which determined manslaughter was most appropriate, said Brenda Kielty, a spokeswoman.

Unlike murder, which generally involves an intentional act, manslaughter charges are brought when a homicide is caused by reckless actions or criminal negligence.

The charge is extremely unusual because of the defendant’s age.

The state doesn’t have data on the youngest person ever to be charged in a homicide in Maine, Kielty said. But it’s been at least 25 years — and possibly longer — since someone so young has been charged with manslaughter or murder in the state, officials said.

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Nonetheless, such cases aren’t unprecedented.

In January, a 10-year-old was taken into custody after a 12-year-old was stabbed to death in El Cajon, California. And a Florida boy was convicted in 2001 of killing a 6-year-old playmate when he was 12.