FITCHBURG -- The investigation into the disappearance of a 5-year-old Kimball Street boy is being treated as a homicide and the boy's mother and her boyfriend have been arrested, Worcester County District Attorney Joseph D. Early said Monday.

Jeremiah Oliver of 276 Kimball St. was last seen by relatives Sept. 14.

Early's office was notified Friday of a potential missing child in the city. Through the course of the investigation, charges were filed against the boy's mother and her boyfriend.

Elsa Oliver, 28, of 276 Kimball St. was arrested Monday on two counts of reckless endangerment of a child and two counts of accessory after the fact of a felony (assault and battery with a dangerous weapon).

She is scheduled to be arraigned in Fitchburg District Court on Tuesday.

Alberto L. Sierra, 22, of 164 Meadowbrook Lane, was arraigned Monday in district court on two counts of assault and battery with a dangerous weapon (knife), and two counts of assault and battery on a child causing bodily injury.

He is being held without bail while awaiting a dangerousness hearing Thursday.

Sierra is not the child's father, according to Early, and his office does not know where his biological father is.

Oliver's other two children have been place in the custody of the Department of Children and Families. Early declined to discuss whether there were any signs the other two children were abused or neglected.

"We've asked the mother to produce the child and if he is not produced, there will be a contempt hearing and that would be a public hearing," Early said by phone Monday.

Anyone with information about the boy is asked to call the State Police detectives assigned to the District Attorney's office at 508-832-9124 or Fitchburg Police at 978-345-9648.