For two years a St. Paul family allegedly posted hundreds of online ads selling sex with several women, some as young as 15, who were coerced, threatened and assaulted into a life authorities described as "modern day human slavery."

The family preyed on young vulnerable girls — girls diagnosed as bipolar or mentally challenged. The suspects' checkered pasts (one was required to register as a sex offender for five years) placed them on police radar for years, but it wasn't until recently that the scope of their alleged sex trafficking ring was exposed and dismantled by a joint effort of St. Paul police, the Ramsey County attorney's office and the Women's Foundation of Minnesota.

But the takedown of the Washington family rested on the shoulders of 10 women and girls who stepped forward, authorities said at a news conference Wednesday morning.

"I want to thank these courageous young women and girls who shared their stories with investigators," said Ramsey County Attorney John Choi. "It saddens me for the victims that it took so long for us to get to this point."

Arrested Monday and charged with various sex-trafficking crimes are Otis D. Washington, 29; his brother Antonio D. Washington-Davis, 26; the brothers' uncles Calvin R. Washington, 49, and Robert J. Washington, 55; and Elizabeth Ann Alexander, 25, the mother of Antonio Washington's children.

Authorities and advocates said the six-month investigation that led to their arrests signals a change in how law enforcement officials are approaching sex trafficking cases and victims, opting for more interagency cooperation while bringing advocates into the fold. Another key, they said, was a "victim-centered" approach that created a safety net for women and girls.

"Trafficking cases are so hard to crack down on," said Kim Borton, director of programs at the Women's Foundation of Minnesota. "This is 10 victims. Imagine the extent of which this is going on in all corners of Minnesota."

The county attorney's office typically charges four to seven defendants a year for sex trafficking or promoting prostitution. In 2012, the office charged 12, Choi said, adding that most victims are under 18.

The sex trafficking in this case spanned from September 2010 to July 2012, Choi's office said, and victims were trafficked as far away as Ely. The Minnesota Bureau of Criminal Apprehension (BCA) connected much of the electronic data that tied the case together.

As part of their enterprise, the brothers used eight e-mail addresses, 30 phone numbers and more than 100 credit-card accounts in placing "hundreds of ads on adult-oriented websites such as," the complaint read.

One woman reported being forced to turn 20 tricks on some nights. Some of the women said they were physically and sexually assaulted by the sex traffickers.

According to the charges: A woman e-mailed police in October and said her 15-year-old granddaughter had been targeted in July by a group of sex traffickers in the 600 block of E. Hawthorne Avenue, where "a number of women [were] being sold for sex." Robert Washington lives at this residence.

When the 15-year-old and an 18-year-old friend were brought to the home by the brothers, one of them told the 15-year-old that her friend was going to serve as a prostitute and tried to persuade the younger girl to do the same. They saw several young women on phones talking with ad respondents about cost and location.

At one point, Otis Washington made sexual advances toward the 15-year-old, but she grabbed his cellphone and called police. Officers were told there had been a fight, and the 15-year-old went home.

The defendants are known to police. Otis Washington pleaded guilty to fourth-degree criminal sexual conduct in Ramsey County District Court in 2002, and was ordered to register as a sex offender for five years.

In 2010, Antonio Washington pleaded guilty to promoting prostitution in a 2009 case in which he trafficked two sisters, ages 18 and 19. According to charges, one sister told police she turned 200 tricks in five weeks, earning about $12,000 to $16,000 that was all given to Antonio Washington. The woman said he gave her about $100 in that time.

Alexander, who was dating Antonio Washington, collected the money, made appointments and was also sleeping with men for money, the complaint said.

Antonio Washington was sentenced to 90 days of confinement and supervised probation until 2025.

Alexander was convicted in 2012 of prostitution in Hennepin County, and was sentenced to 90 days of confinement and one year probation.

Neighbors said they recognized Robert Washington, who lived in the duplex on Hawthorne.

A neighbor who declined to provide her name for safety reasons said she noticed a lot of people, mostly men, going in and out of the house. She also said a pair of Cadillacs would leave and come back a lot, especially at night.

Records indicate that police have been called to the house numerous times within the past few years, responding more than a dozen times in 2012 for fights, possible thefts and other issues.

On three separate occasions in 2010, one victim told police she was being prostituted by Antonio Washington, averaging five or six men and $250 a night. St. Paul police Sgt. John Bandemer, head of the Gerald Vick Human Trafficking Task Force, said that police were aware of those incidents, and that a grant in 2012 from the Women's Foundation of Minnesota helped them identify gaps in their work on sex trafficking cases.

Police and the county attorney's office will create a "tool kit" this year to better tackle such cases, rolling out new investigative and prosecutorial protocol in Ramsey County before launching it as a statewide model.

Authorities declined to speak in more detail about what they've identified as problems and possible solutions, citing a May human trafficking conference where Choi will outline the effort.

The recent case is unusual because of the number of victims and the conspiracy involved, Choi said. "We're at the tip of the iceberg, what you saw today," he said. "The problem is much deeper."

Star Tribune staff writer Nicole Norfleet contributed to this report. Chao Xiong • 612-270-4708 Twitter: @ChaoStrib Paul Walsh • 612-673-4482