An ex-convict accused of strangling his girlfriend, her three daughters and her son also sexually abused two of the children, prosecutors alleged in a court document filed Tuesday.

Prosecutors said they plan to seek the death penalty against Joshua Steven Durcho, 26, for the Jan. 12 killings of Summer Garas, 25, and her children. The bodies of the children were found stacked in a bathtub, while their mother's body was on a bed in an adjacent bedroom.

In court Tuesday, Durcho quietly responded to several questions from the judge, who entered a not guilty plea on Durcho's behalf and set a tentative trial date for April 2010.

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In the new filing, prosecutors allege Durcho sexually abused Garas' eldest daugher, who was days shy of her 8th birthday, "at or near the time" that he killed her. There also was evidence that a 7-year-old daughter was sexually abused, the document states. It does not say whether the sexual abuse took place before or after the girls' deaths.

Assistant District Attorney Paul Hesse said prosecutors have no plans to file additional sexual assault charges against Durcho.

Durcho's court-appointed attorneys have not responded to several requests for comment.

Prosecutors outlined several aggravating factors to warrant seeking the death penalty, including that the killings were especially heinous, atrocious or cruel, that Durcho allegedly killed the victims to avoid arrest or prosecution and that he poses a continuing threat to society.

According to the document, Durcho killed Garas to keep her from contacting police about two outstanding warrants for failing to appear in court on charges of driving under the influence and drug possession.

Durcho, who previously served prison time for possession of marijuana, had vowed never to return to prison, prosecutors said.

In an interview with The Associated Press shortly after the killings, Garas' mother, Susan Rust, said her daughter was planning to break off her relationship with Durcho.

On Tuesday, James Franklin, a step-grandfather of one of the slain children, said family members are preparing for a long and difficult legal process.

"I never imagined I would go to the cemetery to visit the graves of my grandbabies on their birthdays," Franklin said as he choked back tears.