Police officer Daniel Holtzclaw worked the evening shift, from four in the afternoon until two in the morning, patrolling the northeastern part of Oklahoma City. Between February and June he allegedly sexually assaulted at least seven women while on duty, including a 57-year-old grandmother who says she was forced to give Holtzclaw oral sex after he pulled her over. According to police chief Bill Citty, Holtzclaw coerced the women, all of whom were black, into sexual acts by threatening to arrest them.

“They’ve pretty much got power in the palm of their hand. And it’s your word against theirs,” one resident of the neighborhood Holtzclaw patrolled told a reporter. Another said that rumors of the assaults had been circulating for weeks.

News of Holtzclaw’s arrest last Thursday was overshadowed by the police brutality occurring in Ferguson, Missouri. The killing of Michael Brown, an unarmed black teenager, and the crackdown on people protesting his death highlighted endemic problems of racial profiling, brutality and militarization within American law enforcement. Several writers pointed out after Brown’s death that women of color are often left out of these stories of police violence. Sometimes that violence is lethal. In many other cases it’s sexual in nature.

“It’s a huge problem,” said Philip Stinson, a professor at Bowling Green State University and the principle investigator for a Department of Justice–funded research project on police integrity, about sexual assault by police officers. “There are many opportunities for someone, if they were a predator, to engage in crimes of sexual violence that they wouldn’t otherwise be able to do because of the power and authority they have [as a police officer].”

It’s hard to tell exactly how big the problem is, because few people are collecting data. Researchers have to rely on arrest reports and press accounts, which leave out unreported or unprosecuted cases. But even that limited evidence suggests sexual assault is a significant issue in police forces, as The American Prospect and Truthout have reported. According to the Cato Institute, more than 9 percent of reports of police misconduct in 2010 involved sexual abuse, making it the second-most reported form of misconduct, after the use of excessive force. Comparing that data to FBI crime statistics indicates that “sexual assault rates are significantly higher for police when compared to the general population.”

Many cops arrested for sexual misconduct are, like Holtzclaw, repeat offenders. Stinson said that multiple victims of the accused often come forward once a case against an officer is publicized. Others never report the crimes against them, making justice elusive. “Sexual assault is one of the most underreported crimes, and against police officers it’s probably even less reported,” said Jen Marsh of the Rape, Abuse and Incest National Network. In Oklahoma City, several of Holtzclaw’s victims were only identified once investigators began tracing his contact with the public while on street patrol.

As Samuel Walker wrote in his 2002 report “Driving While Female,” officers who commit sexual crimes are often considered “rogue,” in contrast to officers who engage in department-sanctioned forms of racial profiling like stop and frisk. But the Department of Justice was sufficiently concerned about “recurring accusations of sexual offenses implicating law enforcement officers,” that it underwrote an executive guide with policy recommendations for stamping out sexual misconduct in police forces. Though the guide was completed in 2011, it’s not clear if it actually led to changes within individual police forces. The International Association of Police Chiefs, which created the guide, did not return a request for comment. Last year the director of IACP’s research center told The American Prospect, “We think there’s a good-faith effort by police departments out there to be more accountable,” but offered no supporting evidence.

Meanwhile, sexual misconduct is a persistent problem in some police departments. The Federal Bureau of Investigation is examining a variety of allegations against officers from the San Diego Police Department, many of them having to do with sexual abuse. One officer is accused of coercing sexual favors from women he’d stopped for driving while intoxicated. Several women accused another officer of groping them while conducting pat downs.

According to U-T San Diego, many of the alleged victims of San Diego police officers were sex workers, homeless, intoxicated, mentally ill or had criminal records. On a national scale, the lack of data makes it hard to track the demographics of victims of sexual misconduct by police. Stinson’s research suggests that about half are under the age of eighteen. According to a 2007 report prepared for the United Nations Committee on the Elimination of Racial Discrimination, “rape and sexual abuse by police [in the United States] are primarily reported by women of color.”

“What we’ve seen in sexual assaults committed by law enforcement is that they’re targeting victims seen as vulnerable or ‘less credible,’ whether they’re engaged in sex work or are committing a crime. A police officer uses that as a way to control the victim,” said Marsh.

Better data collection would be a small start to tackling the problem. Another would be to mandate that states share information about cops who’ve been fired or resigned after allegations of sexual misconduct. There’s even a database set up for it: the National Decertification Index, which lists officers who’ve lost their certification. But contributions are voluntary and with only thirty-seven states participating, it’s a leaky sieve.

One legislative reform that’s been proposed is extending the 2003 Prison Rape Elimination Act, which requires prisons and jails receiving federal money to enforce a “zero tolerance” policy towards sexual violence by staff, to apply to anyone who is stopped or detained by law enforcement. Stinson said that requiring police officers to change partners and supervisors between their shifts could help, as could the use of body cameras, although he cautioned that there were still important privacy concerns to consider regarding the use of recording equipment. Other advocates point to the small proportion of female cops as a problem; according to the International Association of Police Chiefs, less than 14 percent of police are women.

More challenging, but just as necessary, are changes in the way that law enforcement works in communities of color, with sex workers, and others at risk of police violence. It will be difficult to encourage more women to report cops who commit sexual crimes if the victims don’t have reason to believe that their assailants are truly “rogue” actors, and that other members of the department will take their complaints seriously. “Who are they going to call?” Penny Harrington, the former chief of the Portland, Oregon police and founder of the National Center for Women and Policing, has asked. “It’s the police who are abusing them.”