The girl was charged Thursday with manslaughter in the death of 3-month-old Brooklyn Foss-Greenaway of Clinton, 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. She is due in juvenile court in October.
Brooklyn's mother, Nicole "Nicki" Greenaway, said the 10-year-old was the baby sitter's daughter. Authorities told Greenaway her baby had ingested medication to treat attention-deficit disorder and been suffocated, she said, adding that she also saw bruises on her baby's body.
"I feel a little bit of relief that they're charging her (the sitter's) daughter at this point, but the mom really needs to be responsible. She's the one I left my daughter with," Greenaway said.
Brooklyn was in the baby sitter's care overnight 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 the child was unresponsive.
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.
Stephen McCausland, spokesman for the Maine Department of Public Safety, said he couldn't comment on whether charges could be brought against the sitter.
A person who answered the phone at a listing for the sitter said Thursday evening that she was not available. The Associated Press is not naming the sitter because it could make known the juvenile suspect's identity.
In Maine, the death of any child under age 3 triggers an automatic investigation. McCausland said detectives uncovered some "troubling signs" before the state medical examiner declared the baby's death a homicide Wednesday. Investigators would not provide further details on the cause of death.
The 10-year-old girl was in the custody of the Maine Department of Health and Human Services when a summons was delivered to her attorney on Thursday, McCausland said.
Her attorney didn't immediately return a call from the AP.
The charge was filed after detectives consulted with the attorney general's office, which determined manslaughter was most appropriate, said spokeswoman Brenda Kielty.
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.
Maine doesn't have data on the youngest person ever to be charged in a homicide in the state, Kielty said. But it has been at least 25 years -- and possibly longer -- since someone so young has been charged with manslaughter or murder in Maine, officials said.
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, Calif. And a Florida boy was convicted in 2001 of killing a 6-year-old playmate when he was 12.