Three people are facing charges of interfering with the investigation into the disappearance of 5-year-old Jeremiah Oliver, the Worcester district attorney’s office said this afternoon. Those charged include the brother of the man already charged with assaulting the young boy.

The three people were charged under the witness intimidation statute, Worcester District Attorney Joseph D. Early Jr.’s office said in a statement. The statement did not say what the three had allegedly done.

Christian Sierra, 21, Cailey Thibault, 23, and Ashley M. Cormier, 24, were arraigned today in Fitchburg District Court, prosecutors said.

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Christian Sierra is the brother of Alberto L. Sierra, 22, the boyfriend of Oliver’s mother, who is facing charges that he attacked Oliver.

Sierra and Thibault were arrested Friday night. Cormier was arrested today. Judge Andrew Mandell set $2,000 bail for Sierra and set bail at $2,500 each for Thibault and Cormier. He slated a further hearing for April 15.

Jeremiah Oliver was last seen by relatives Sept. 14. Prosecutors said they learned in December that he might have gone missing.

Mystery still surrounds the whereabouts of the boy. Authorities fear he is dead.

No one has been charged with his murder.

But Elsa Oliver, 28, the boy’s mother, faces 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) in the case.

And Alberto Sierra is facing two counts of assault and battery with a dangerous weapon and two counts of assault and battery on a child causing bodily injury, prosecutors said.

The case has prompted scrutiny of the Department of Children and Families, the state child protection agency, which was supposed to be checking on Jeremiah but had not visited the boy for months.