Civil rights lawyers have uncovered evidence of widespread sexual abuse of detainees at immigration detention facilities across America in what they say is the systemic violation of a particularly vulnerable population.

The American Civil Liberties Union has discovered details of 185 official complaints of sexual abuse that have been made against the main immigration authority in the US, Immigration and Customs Enforcement (ICE) since 2007. The ACLU, which obtained the details through freedom of information requests, believes that is just the tip of the iceberg.

Most of the complaints involve allegations of abusive sexual advances made by male detention officers against female detainees. What struck ACLU researchers was that the claims were not focused on a few rogue facilities or isolated cases, but showed a pattern of problems that extended right across the country.

"The evidence points to a systemic problem," said David Shapiro, an ACLU lawyer in its national prison project. "The sheer fact that the allegations are so widespread is cause for alarm."

Shapiro added: "Just one case of sexual abuse is intolerable – there should be zero tolerance of it – but our findings show this goes far beyond that."

A map compiled by the organisation from its findings shows that 22 of the 50 states in the union have had allegations of abuse lodged against them, with California, Arizona and Texas standing out in the numerical league table.

Texas in particular has faced a heavy weight of accusations, with 56 complaints lodged against it. That is in part explained by the concentration of detention facilities in the state, though civil rights lawyers say that the striking number of allegations arising in Texas bears further investigation.

Three of the most egregious cases to have emerged in recent years all occurred in Texas, at the Don Hutto Family Residential Center. The facility is run on behalf of ICE by America's largest private prison company, Corrections Corporation of America.

The ACLU in Texas on Wednesday issued a civil lawsuit against the company and ICE officials on behalf of three women who were sexually assaulted while in custody at Hutto having appealed for asylum in the US.

The women were all abused by Donald Dunn, a guard at Hutto who has pleaded guilty to five counts of official oppression and unlawful restraint involving assaults on five women prisoners. He is also facing sentencing on other criminal civil rights violations.

Court papers lodged in the ACLU action give details of what happened to the three women, whose full identities are being withheld to protect their safety.

"Raquel" was arrested by immigration officers as she tried to cross the Mexico border, having faced death threats in her native central American country after her husband was murdered. She was transferred to Hutto, and while her application for asylum was being processed she was allowed out on bail.

But she was attacked by Dunn while he was transporting her to the airport upon her release, as she relates in the court documents: "I thought he was going to kill me. I thought I should have stayed in my home country if my life was going to end like this, because at least I would have had more time with my children. I kept yelling, I was crying."

The number of undocumented immigrants who have been arrested in the US and put into detention facilities pending deportation has increased dramatically in recent years. President Bush introduced the Secure Communities scheme shortly before he left the White House, and it was then extended under the Obama administration, to the disappointment of many civil rights campaigners who see it as a form of harassment of immigrant groups.

As a result, detention of immigrants has become something of a boom industry in America, with 250 facilities now operating across the country where more than 30,000 detainees are held at any one time, more than 60% of whom were of Mexican origin.

The documents released to ACLU under freedom of information are heavily redacted, but give an insight into the kind of abuse complaints that are regularly being raised. One complaint alleged that they were made to strip naked in front of two other individuals while at a detention centre in Orange County, California. "It was also alleged that the complainant was laughed at mockingly in an intimidating fashion."

Other complaints ranged from inappropriate touching to forceful fondling and rape.