An 11-year-old autistic Middleboro boy’s claim that he was sexually assaulted by another youth inside his foster home was dismissed as “consensual” sex by child welfare officials charged with protecting him, in a stunning case the boy’s family and school officials charge was plagued by missteps by the embattled Department of Children and Families.

“I want DCF to be held accountable,” said the boy’s biological father, whose name the Herald is withholding so as not to identify his now 16-year-old son. “It was hard to think that nobody did their job … or that they all failed miserably and my son had to pay the ultimate price for that — losing his innocence. It will affect him for the rest of his life.”

The case — one of several on the radar of lawmakers preparing to haul in DCF officials for questioning on agency failures on Thursday — is detailed in competing narratives of the autistic boy and officials at DCF and Massachusetts MENTOR, a state contractor hired to oversee the foster home, who brushed it off as a “consensual and experimental” encounter.

The allegations in the 2009 case are outlined in a raft of reports, letters and other documents kept by the boy’s biological father and provided to the Herald.

The father and school officials also maintain that the autistic boy may have been taken back to the foster home just days after the alleged incident.

Further, DCF officials were more motivated to cut costs when they decided to move the boy from residential treatment to foster care where the alleged incident occurred, according to a former social worker.

A school official, in a fiery letter to then-DCF Commissioner Angelo McClain, called it “one of the most abhorrent situations I have encountered in my career.”

“The alleged sexual abuse of an 11-year-old boy has been completely minimized in this case,” said John C. Randall, president and CEO of Amego Inc., the school and residential treatment center the boy was attending. He said DCF staff “did not respond properly, but exacerbated the situation” by emphasizing their rocky history with the boy’s biological father, not the incident at hand.

“I am a calm, rational and direct person who was completely frustrated in my communications with your staff,” Randall wrote. “I can’t imagine where I would have been emotionally if it had been my son involved in this case.”

According to DCF investigative reports, police reports and letters, the boy — then 11 but with the cognitive ability of a 5- or 6-year-old, his father told police — was allegedly sexually assaulted by another youth in the foster home while the parents slept during the early morning hours of May 2, 2009.

Officials at Massachusetts MENTOR, a state contractor that supervised the foster care, said they immediately removed the 11-year-old from the home, but they didn’t take him to the hospital because both boys initially said the incident entailed only touching and rubbing — actions officials described as “consensual and experimental.”

But two days later, the boy told a teacher at Amego he was “harassed” into sexual contact, prompting school officials to rush him to a hospital to be examined.

Both the autistic boy and the other youth described the contact as sexual in nature — and that it had happened three to four times — but DCF investigators, in wrapping their probe, indicated it was consensual, writing in a report that “the boys took proactive steps to avoid parental detection.”

DCF investigators never interviewed either of the boys, citing a potential criminal case, and said doctors found no evidence of “tears or bruising” on the autistic boy, though, as one social worker pointed out in the probe, the examination didn’t come until days afterward, according to an investigation report.

“The children were opportunistic regarding their actions,” investigators wrote in June 2009, when they also found the foster parents’ supervision to be “appropriate.”

Paul Cataldo, executive director of Massachusetts MENTOR, said in a statement that the agency has worked with the foster home since 2001, and the parents have provided a “warm, loving home for more than 20 children in need.” He said in all abuse cases, MENTOR follows DCF protocol, including “seeking medical attention for the child involved, as appropriate based on the facts of the case,” and that new protocols were put on the home as a result.

But the boy’s father noted that in the years after the incident, his son’s behavior was marked by violent outbursts and sexualized behavior, and he had to be hospitalized in 2010 and returned to a residential program in 2011. “They called it ‘consensual’ sex … It’s pretty frightening that they thought it was OK,” the father said.

He and Randall also accused officials of letting the foster father bring the boy back into his home in the days afterward to have dinner.

Cataldo said the foster father helped shuttle the boy from his temporary home and to a school bus stop for four days until the plan was stopped given the DCF investigation.

Efforts to reach the foster father were not successful.

Local police, who first received notice of the incident four days after the alleged incident, investigated but closed their case in 2010 after the probe stalled.

DCF spokeswoman Cayenne Isaksen said, “The Department works closely with providers when issues may arise to mitigate any problems that may exist and to ensure that they can, and are, providing the optimum level of care to our children.”