The U.S. Department of Justice says it has uncovered “substantial evidence” that sexism in a Montana county Attorney’s Office has led to prosecutors dropping rape cases and belittling victims.

In a 20-page letter sent to Missoula County Attorney Fred Van Valkenburg, federal attorneys said rape victims had been “treated with disrespect, not informed of the status of their case, and re-victimized by the process.”

For example, federal officials said, when one mother asked a county attorney why an adolescent boy who assaulted her 5-year-old daughter had only received two years of community service, the attorney responded, “Boys will be boys.”

Federal investigators added that Missoula prosecutors had failed to pursue “nearly every case” in which a mentally or physically disabled woman claimed sexual assault, “even in cases where there was evidence such as a confession or incriminating statements by the perpetrator.”

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Under the threat of a possible federal civil-rights lawsuit, the letter urged the county attorney to meet with Justice Department officials to reach an agreement to reform his policies.

In response, Van Valkenburg told the Los Angeles Times that the Department of Justice’s letter sent Friday was “replete with falsities.”

He added that the public letter was a political maneuver to “poison the well” after he filed a federal lawsuit against the Department of Justice last week, alleging that his office is being wrongfully investigated. He has asked a judge to block the federal investigation.

The Department of Justice letter came after nearly two years of inquiries by federal civil-rights attorneys into allegations that officials in Missoula County systematically and unconstitutionally mishandled potentially dozens of sexual-assault cases.

The investigation began in 2012 after allegations that rapes and gang rapes of University of Montana students — in some cases, involving university football players — were wrongfully not prosecuted.

In the Friday letter, federal investigators said they found that only 14 of 85 of the cases police referred for prosecution between 2008 and 2012 — about 16 percent — resulted in charges brought by the county prosecutor’s office.

In May, the Missoula Police Department and the University of Montana in Missoula agreed to reform their practices in handling such cases after similar federal inquiries. Federal officials said the Missoula County Attorney’s Office, however, refused to cooperate.