Feb. 2, 2012 -- A Texas teenager will spend at least the next decade of his life in prison for shooting his sister in the head four times.
Nate Anderson, 16, pleaded guilty to first-degree murder today in a Tyler, Texas, courtroom. He was sentenced to 20 years in Texas state prison for the September 2010 slaying of his sister Amanda Anderson, 19, in their family home, but will be eligible for parole after he has served 10 years.
During the court proceedings, Anderson said only "yes" and "no" to the court's questions. His parents, Mark and Stacie Anderson, were allowed to submit affidavits instead of speaking in court.
"As a mother who loves both of her children, this has been the most difficult ordeal I have ever experienced," Stacie Anderson wrote in her statement.
The Andersons said they understood the importance of justice for both of their children.
"I believe that the district attorney is pursuing what he believes is justice for Amanda. As Amanda's father, I appreciate the work of the district attorney," Mark Anderson wrote.
Both parents said they have spoken to their son and forgive him for killing their daughter.
District Attorney Matt Bingham expressed sympathy for the Anderson parents after the sentencing.
"My prayers are with the Andersons because they have essentially lost both of their children," Bingham said. "I can only imagine how difficult this has been for them."
Nate Anderson, who was 15 at the time of the murder, told police he went for a jog near his home. When he returned, he said he stood in the doorway and saw his sister had been shot twice.
The case didn't fit the usual mold of a botched robbery or jealous boyfriend and they became suspicious of Nate's story.
Detectives matched the caliber of the weapon to one Nate had recently been given by his father for target practice.
Microscopic traces of blood were found on the boy's shoes and clothes -- splatters that would have only come from being in the room at the time of the murder.
Nate Anderson was arrested two months later at the Tyler Gospel Chapel where he attended school.
Anderson's family denied he was the murderer until he confessed to a psychologist hired by the defense in January of last year.
The teen, who has been in solitary confinement for the past nine months as a safety precaution, will serve his time in an adult prison. His defense attorneys told ABC News he will likely be grouped with young adults at first.
ABC News' Santina Leuci contributed to this report.