FARGO - The father of a teenager accused of killing his sister in Fargo has written a letter to the judge in the case, asking him to reconsider a plea agreement and spare his family a trial.

"It's my beautiful daughter that has been murdered by my son," the Rev. Scott Carlson wrote in a letter to Judge John Irby. "I have heard his confession. I know what he did. I know the horror of my daughter's last moments on this Earth. That is an image that will remain with me for a long time."

Sergei Carlson, 16, is charged with murder and a deviate sexual act in the death of his 16-year-old sister, Whitney Carlson. Irby rejected a plea agreement in May that called for a sentence of 30 years in prison.

In his letter to Irby, Scott Carlson of Sun Prairie, Wis., said the judge's rejection of the plea deal made him feel as if he had been kicked in the stomach.

"I'm asking you to please reconsider this judgment," Carlson wrote.

Carlson told The Associated Press on Thursday that he decided to make the letter public because he had not heard from the judge. Defense attorney Mark Beauchene declined comment. Rod Olson, the Cass County court administrator, said Irby did not feel it would be appropriate to discuss the letter.

Prosecutors would not talk about the specifics of the letter.

"We're mindful of Sergei Carlson's right to a fair trial. We want to make sure that we preserve that right," said Reid Brady, Cass County prosecutor. "We don't fault the father, either. We wouldn't criticize him at all. At the same time, we respect the judge's decision."

Authorities said Sergei Carlson told police he strangled his sister in her bedroom with his hands, put pillows over her face to muffle her sounds, then had sexual contact with her.

The two-page letter from Scott Carlson, dated June 1, was included in the case file but sealed from public view. Carlson faxed it to media outlets Thursday, along with a cover letter explaining his frustration with the judge's ruling.

"The sad news is that because of his decisions, not only does our family continue to endure a lengthy process that could have and should have been done a month ago," Carlson said. "But as we move closer and closer to a trial, the reality is this will continue for a long time."

Carlson said he is upset that more details of the crime will become public during a trial that will "come at a great cost" to the taxpayers of North Dakota.

Beauchene said in court documents that Irby would only accept a sentence of life in prison with the possibility of parole. Beauchene said that showed bias toward his client and he asked for a new judge. Irby refused the request, but ordered the trial moved from Fargo.

Sergei Carlson, called Isaac by family members, was adopted from Russia when he was 7 years old and moved with his father to Sun Prairie in 2002. Whitney Carlson lived with her mother in Fargo. Sergei, Whitney and three other sisters alternated between parents for summers and holidays.

Scott Carlson concluded the letter to Irby by telling the judge how difficult it has been to try explain new details of the case to his four daughters.

"Words cannot express the depth and pain and grief I feel in the death of my daughter, Whitney. I miss her greatly," Carlson wrote.

"But I also know how much I have invested in Isaac, through adopting him and welcoming him into our family. I know the energy I have put in to give him a shot at a life.

"While I certainly want to have him held accountable for what he has done, I also want to give him a shot at rehabilitation and a life," Scott Carlson wrote. "That is why I felt this plea agreement was a good one."