The death in August of a 1-year-old boy who suffered injuries while being cared for by a babysitter in Woburn has been ruled a homicide, authorities said Thursday. Middlesex District Attorney Marian T. Ryan’s office confirmed the ruling in the death of Noah Larson. No charges have been filed, and the investigation is ongoing, a Ryan spokeswoman said Thursday night.

A baby sitter was watching Noah and his twin sister when paramedics were called to the sitter’s home on Aug. 15.

Noah’s mother, Lindsey Keane, previously provided The Boston Globe with a document from the state’s child protection agency that says in part, “the current facts point to Noah’s fatal injury having been sustained while in the home of his babysitter.’’

Keane could not be reached for comment Thursday night.

That same document from the Department of Children and Families noted another disturbing aspect of the case: It said doctors found that both twins had old skull fractures on the back of their heads, likely a week or so old, suggesting abuse occurred prior to Noah’s fatal injuries. The twin sister also had some healing fractures near her ribs and limbs, Keane said. Keane acknowledged that someone, other than the baby sitter, could have had access to her twins while they were at the baby sitter’s one-bedroom apartment. For about a year, the sitter primarily received the twins when they came, but sometimes the sitter’s husband was there, and some other relatives. The sitter also cared for her own toddler while watching the twins, Keane said.

Keane said she had a good relationship with the sitter — a former friend of Keane’s — and paid her $65 a day to care for the twins while she worked part time in office jobs and sales and the twins’ father, Paul Larson, 26, worked as a solar energy surveyor. Prior attempts by the Globe to reach the baby sitter were unsuccessful.

In a lengthy interview earlier this year, Keane told the Globe through tears that she often revisits that moment when she got the call from the sitter saying Noah was unresponsive, and then paramedics were called.

“She was someone we trusted,’’ Keane said in the interview.

Travis Andersen can be reached at travis.andersen@globe.com. Follow him on Twitter @TAGlobe. Patricia Wen can be reached at patricia.wen@globe.com.