ALTAMONT, Tenn. — A Grundy County mother of three remains in jail, now on a charge upgraded to murder, as an investigation continues that could add more counts, said Grundy County Sheriff Brent Myers.

Tracy City resident Shannon Marie Comeau, 21, was jailed last week on aggravated child abuse charges stemming from injuries sustained by her 7-week-old baby. She now faces felony murder charges after the infant died Saturday, the sheriff said during a news conference here Monday.

Ms. Comeau, who’s held on $100,000 bond, faces arraignment Wednesday on both charges, he said. The baby’s father, who does not live with Ms. Comeau, has not been charged.

Sheriff Myers said there are allegations involving Ms. Comeau’s other children, ages 1 and 3 years, and a separate incident involving the baby.

“There’s already been an investigation opened up into the other two children,” he said. “When we get complete with that investigation, there could be other charges.”

He said investigators found other damage to the baby’s brain believed to be from a separate injury than the one that killed the infant, he said.

Investigators would have to rely on medical experts to determine what caused the baby’s earlier brain trauma, Sheriff Myers said.

Assistant District Attorney Steve Strain said the prosecution’s case generally is the same.

“The underlying facts are not different except that the child has succumbed to its injuries,” Mr. Strain said.

Some residents of Tracy City said they are confused by the events.

Tim Kilgore said he worked periodically with Ms. Comeau over the last couple of years at the Hardee’s restaurant in Tracy City.

“She was a good person, but I can’t say I know how she was as a mother,” said Mr. Kilgore, 30. He said Ms. Comeau seemed to care for her children when they were around.

Norma Rollins, a resident of nearby Coalmont who was shopping in Tracy City on Monday, said, “Everybody on this mountain takes care of their children.”

But she was baffled by the baby’s death.

“I don’t know. When something happens to a child, it’s a real tragedy,” she said.