The agent in question denied the allegations, including the sisters’ claims that he touched their genitals. He insisted he had only fingerprinted the sisters before taking them back to their cell.

Investigating the case came down to the sisters’ word versus the agent’s. And as in dozens of similar cases, government investigators sided with the agent.

Advocates say the case — outlined in a report compiled by internal investigators — shows the kinds of hurdles detained immigrants face when they make claims of misconduct, even when they come forward immediately, as the sisters did.

“These women were actually, for lack of a better word, lucky that their case was investigated,” said Christina Mansfield, co-founder of the advocacy group Freedom for Immigrants. “They are in the extreme minority in that regard.”

The sisters, ages 17 and 19, entered the U.S. without legal permission in July 2016, several days after leaving their home village in Guatemala. They were detained by Border Patrol agents shortly after crossing the border.

The Associated Press received a redacted copy of the investigative report through the Freedom of Information Act. It shows that investigators determined that the sisters’ allegations could not be substantiated due to a lack of physical evidence.

The station where the sisters were detained did not have cameras in the booking area. The room where the sisters say they were taken, later described as a supply room or a closet, wasn’t processed for fingerprints because the sisters said they didn’t touch anything. And the agent in question said he was alone with the sisters due to manpower shortages, the report says.

Immigration advocates say the Department of Homeland Security’s Office of Inspector General, which reviewed the sisters’ case, rarely recommends action against officers. A study by Freedom for Immigrants found that between January 2010 and July 2016, the inspector general received 84 complaints of coerced sexual contact against U.S. Customs and Border Protection, which includes Border Patrol. The inspector general opened just seven investigations, none of which are known to have led to charges, according to the study, which was conducted by examining government records.

The study found a similarly low number of cases were investigated by the inspector general for detention facilities operated by U.S. Immigration and Customs Enforcement.

CBP would not directly address the sisters’ case or whether it disciplined the agent involved. The agency said it was committed to treating detainees with “professionalism and courtesy.”

Immigration authorities detain and process thousands of people every month who cross the U.S. border without permission. Border Patrol chief Carla Provost said in a recent interview that her agency takes any allegations against any of its 19,000 agents “very, very seriously.” Provost previously led CBP’s internal affairs division, which also investigates allegations of employee wrongdoing and that the agency has taken steps to strengthen in recent years.

“They are very few and far between,” she said, speaking generally about misconduct complaints.

The sisters were taken by agents to the Presidio Border Patrol station and booked into the same holding cell. The agents who detained them went out on another call.

According to the report, a third agent then took the elder sister out of the cell. He told investigators that he processed and fingerprinted each of them before returning them to their cell.

The elder sister told a different story. She said the agent took her into a back room that had a table filled with snacks and drinks. There, she says, the agent told her to lift her sweater and dress. The woman said that after a small photograph fell out of her bra, the agent pulled up her bra. Then, she said, he told her to remove her pants and underwear, then touched her genitals with the outer part of his hand.

The younger sister would accuse the agent of conducting a similar search.

Border Patrol guidelines prohibit male agents to strip-search female detainees “except in exigent circumstances,” and only then with another agent present to observe. Under the guidelines, a vaginal cavity search must be done by a medical professional at a medical facility.

When the agent later heard one of the sisters accusing him of misconduct, the report says, he “became upset and told the girl he didn’t make her do that.”

According to the investigators’ report, one agent said that “this is exactly the reason cameras are needed in the processing area.”

The accused agent would speak to investigators three times. The last time, an investigator noted the agent “appeared to be nervous and removed,” and he “had to constantly review a prepared statement” from his initial interview. But a month after the last interview, investigators took their case to a federal prosecutor, James Miller, who agreed with their conclusion that there was a “lack of evidence.”

Miller declined to comment on why he didn’t pursue a prosecution. The agent did not return phone messages from The Associated Press, nor did his attorney, Raymond Martinez.

The sisters were eventually released and went to live with their mother in California. One of the sisters has since sued the U.S. government. Court filings show both sides are now discussing a settlement.

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Lauer reported from Philadelphia. Associated Press writer Colleen Long in Washington contributed to this report.