An infant's broken arm led police to arrest a Union man for allegedly assaulting the whole family.

Public Safety spokesman Steve McCausland says detectives were alerted to the situation after an 18-month-old girl was taken to PenBay Medical Center with a broken arm. As detectives began investigating, they learned that the girl's 5-year-old sister also had been assaulted, as well as the children's mother.

Charged with two counts of Aggravated Assault and one count each of Domestic Violence Assault and Unlawful Sexual Contact is 27-year-old Jessie Tirrell. He is the mother's boyfriend and lived at the Searsmont Road house with the family. Tirrell is being held at the Knox County Jail.