SOUTH PARIS — A teen-age girl who was the state’s star witness at the trials of three youths charged with murdering an elderly farmer pleaded guilty Monday to juvenile charges of felony murder and robbery and was ordered confined to the Maine Youth Center.

Shari Lee Brown, 17, was taken to the South Portland reformatory immediately following a two-hour hearing in which two of the youths she had testified against showed up at the courthouse.

Security was tight in District Court as Judge John Sheldon accepted Ms. Brown’s plea in accordance with an agreement with the state that allowed her to plead guilty to juvenile charges and escape prosecution as an adult.

Assistant Attorney General Gregory Motta told the judge that Ms. Brown had carried out her end of the bargain by testifying earlier this year at the Bangor trials of Jay Snow, Nathan Wade Conley and Christopher Fitch, all 17 and from the Norway-South Paris area.

Separate juries acquitted Conley and Snow, the alleged trigger man, but convicted Fitch of the 1988 gunshot slaying of Lucien Frechette, 82, of Norway, during a robbery the state maintained was carried out to obtain money to buy drugs.

While Motta made no recommendation on the disposition of Ms. Brown’s case, her lawyer, David Grund, appealed to the judge to suspend her confinement at the youth center and permit her to return to Pennsylvania, where she has been living.

Sheldon refused, electing to impose an indeterminate sentence at the reformatory, where Ms. Brown could remain until her 21st birthday.

Taking the stand on her own behalf, Ms. Brown said that while her initial motivation in seeking a plea agreement was self-interest, she decided last February that she wanted most of all to do the right thing.

She said discussions with a counselor that enabled her to confront emotional problems she had been facing played a key role in her changed outlook.

Also speaking on Ms. Brown’s behalf were her mother, Brenda Buck, and the father of her unborn child, Derek Kennedy.

Snow and Conley appeared at the courthouse during Ms. Brown’s hearing. No sentencing date has been set for Fitch, who faces 25 years to life imprisonment.

Defense lawyers had portrayed Ms. Brown as a liar who fabricated the story about the youths’ involvement in order to get back at Conley for dating other girls after he had stopped going out with her.