JACKSON, Miss. — Girls at a Mississippi detention center were sometimes shackled for 12 hours a day and subjected to "horrendous physical and sexual abuse," a youth advocacy group claims in a federal lawsuit.

The Mississippi Youth Justice Project sued the state Wednesday on behalf of six girls, ages 13 to 17, and called for the shutdown of the troubled Columbia Training School.

All the girls involved have mental illnesses and were committed for nonviolent offenses, said Sheila Bedi, the Mississippi Youth Project director.

One of the girls in the lawsuit says she was sexually assaulted. Another says a guard sent her a sexually explicit letter. Others were subjected to excessive use of restraints, a lack of mental health care and other abuses at Columbia, the lawsuit says.

Five of the girls say that in May they were shackled from a week to about a month for 12 hours a day at the detention center.

"At times, Columbia staff failed to 'double-lock' the shackles so that they would tighten around the girls' ankles with every step they took. This caused excruciating pain, and they frequently complained to Columbia staff about the injuries they sustained as a result of the shackling," the lawsuit says.

Julia Bryan, a spokeswoman for the state Department of Human Services, which oversees the school, said the agency had not reviewed the lawsuit and would have no comment on pending litigation. The lawsuit names Gov. Haley Barbour and DHS executive director Don Taylor among the defendants.

After the abuse allegations surfaced in May, Taylor suspended six employees with pay and launched an investigation into the claims. DHS officials have said they will release a report when an investigation is complete.

The center, which houses about 33 girls and has an annual operating budget of $5 million, has been accused of abuse before.

In 2004, the U.S. Department of Justice sued the state over conditions at Columbia and the state's facility for troubled boys, Oakley Training School. Among the accusations were claims that youngsters at Columbia were forced to eat their vomit and were tossed nude into isolation cells.

Mississippi entered an agreement in May 2005 to end the lawsuit. As part of that four-year consent decree, a court monitor oversees progress at the facilities. The latest report, released last month, says some conditions are improving but there are still problems at the schools.

"The fact that these violations persist, is incredibly telling about how broken this system is," Bedi said.

One of the girls in the lawsuit, identified only as a 15-year-old with the initials M.S., said she was sexually assaulted in May by a male guard at the facility.

The guard "proceeded to grab M.S. in an embrace, kiss her, and rub her breasts and genitals with his hands over her clothes," the lawsuit says. He "returned later that same night ... again entered her cell, grabbed her around the waist and attempted to put his hand in her underwear." The girl struggled, and the guard left the room, the lawsuit says.

The girl described the alleged abuse to reporters during an emotional interview Wednesday in which her mother broke down in tears. She said she reported the abuse, but "nothing was done about it."

"They told me I was lying," she said. "They told me that I was wrong for reporting it, that I shouldn't have brought it up."

A 17-year-old girl identified as E.S. told reporters she was shackled in leg irons for three weeks.

"I have nightmares at night. I have scars on my ankles. I cry when I see shackles or handcuffs," she said.

E.S., who is white, said she was discriminated against because of her race and witnessed a guard asking another girl to show her breasts.

It is Associated Press policy not to identify suspected victims of sexual abuse.

State Rep. George Flaggs, chairman of the House Juvenile Justice Committee, said in a statement that lawmakers need to fix the problems at Columbia so the state doesn't end up in court again.

"The time has come to transform Columbia from a facility notorious for abuse and neglect to a facility that contributes positively to the future of all Mississippians," the Democrat said.