HONOLULU — No one will be charged in the violent shaking assault of a toddler that left him with a brain injury nearly three years ago, the Hawaii attorney general’s office announced Wednesday.

Child welfare investigators determined Peyton Valiente was likely injured at his day care, which was operated by the wife of a Honolulu police officer. His parents, Chelsea and Rey Valiente, believed that police connection was the reason no one was ever arrested in the January 2015 assault.

The attorney general’s office told the Valientes the witnesses subpoenaed to testify before a grand jury invoked their fifth amendment right against self-incrimination, the couple told reporters.

“It was difficult to break the news to the parents … that our findings did not result in charges against a perpetrator,” Attorney General Doug Chin said in a statement. Despite multiple witnesses summoned to testify before a grand jury, a suspect couldn’t be identified, he said.

Deputy Attorney General James Walther confirmed witnesses refused to testify.

“They couldn’t move forward,” Chelsea Valiente said. “We’re frustrated we’re disappointed and yet we’re not surprised.”

The couple’s civil lawsuit against the sitter and her now-retired husband is in early stages of litigation, their attorney Chris Bouslog said,.

He added that it’s reasonable the attorney general investigation didn’t yield any charges. “We know that the attorney general conducted quite an extensive investigation into this.”

After Honolulu online news site Civil Beat detailed the family’s ordeal, then-acting Chief Cary Okimoto said earlier this year he had serious concerns and ordered an administrative investigation.

Okimoto apologized to the Valientes in March, saying he was disappointed the case didn’t move as quickly as it should have. Okimoto told the police commission that he personally looked into the case. Witnesses had already secured attorneys before detectives interviewed them, he said.

Police turned the criminal case over to the state.

On Jan. 9, 2015, Chelsea Valiente said, she got a call from the baby sitter saying Peyton was having trouble waking up and had vomited earlier. His breathing was shallow, the woman told the mother.

Valiente, a nurse, told the baby sitter to call 911 and rushed over. When she arrived, “I saw him lying on the floor unresponsive, legs and arms stretched out and stiff,” she said in February.

Doctors found bleeding in Peyton’s brain, according to records provided by the Valientes.

Doctors told Valiente the only way Peyton could have suffered those kinds of injuries would be through forceful shaking. A doctor found finger-like bruising on Peyton’s back, so police and child-welfare services were called and the Valientes temporarily lost custody of Peyton.

A report by child-welfare investigators concluded the Valientes didn’t hurt Peyton and that it most likely happened in the baby sitter’s home. The state Department of Human Services then withdrew its petition for temporary foster custody.

The now-4-year-old boy is prone to head injuries and it’s not clear what other long-term effects will be, Bouslog said.

“Quite frankly we lose sleep on it all the time because we don’t have answers,” Chelsea Valiente said. “One day when Peyton is old enough to understand the extent of his injuries I have no answers to give him, other than this is what happened to you. But I have no answer as to who did it.”