BOSTON (WHDH) - A Chelsea foster dad who allegedly raped and abused two young girls who were in his care was arraigned this week on new child sex charges, authorities announced Friday.

Michael Diaz, 34, was arraigned in Suffolk Superior Court on 13 charges, including four counts of aggravated rape of a child, six counts of indecent assault and battery on a child, one count of dissemination of matter harmful to a minor, one count of posing a child in a state of nudity and one count of possession of child pornography, according to Suffolk District Attorney Rachael Rollins.

All of the charges stem from his treatment of his pre-teen foster daughters, ages 12 and 11, who were in his care between 2017 and 2019, Rollins’ office said in a news release.

“This is the most evil betrayal of trust I can imagine,’’ Rollins said. “I am revolted by the charges, not only as district attorney but as a licensed foster parent and an emergency foster care provider.”

Diaz was a Department of Children and Families approved foster parent to the two victims when the alleged sexual abuse occurred. DCF first became aware of the allegations in early 2019 and the children were immediately removed from his home.

Prosecutors say both girls had been instructed by Diaz to keep his behavior a secret, and they did so for a while. Diaz also allegedly gave the 12-year-old victim two cellular phones and continued to communicate with her after she was removed from his foster home.

In June 2019, the 12-year-old victim told her biological mother that her foster father had sexually assaulted her on multiple occasions, prosecutors said. She also stated that she was concerned about the wellbeing of her 11-year-old foster sister.

Chelsea police arrested Diaz after interviewing both victims in July 2019.

Forensic examinations of Diaz’s phone and the victims’ phones “produced corroborative photographic evidence” of child sex crimes, according to prosecutors.

Diaz was initially arraigned in July 2019. He later posted $25,000 bail and was released. In December 2019, a Suffolk County grand jury indicted him on additional charges.

“Foster parents are supposed to provide the children in their custody with the safety, support, stability, and care they need in order to move forward and recover from the trauma many have experienced prior to the Department of Children and Families’ involvement,” Rollins said. “That an individual would further harm the most innocent and vulnerable among us represents unfathomable depravity.”

Diaz is due back in court at a later date.

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