Raleigh, N.C. — A DNA swab taken from the bed sheet that state school board member Kathy Taft had been sleeping on when she was attacked two years ago could not have matched anyone but her accused killer, two analysts testified Friday in his first-degree murder trial.

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Michelle Hannon, a forensic biologist specializing in DNA at the North Carolina State Crime Laboratory, said that there was a 1 in 1 trillion chance that the sperm fraction belonged to someone other than Jason Keith Williford. She noted that the world population at the time of testing was about 6.8 billion people.

Advanced testing by private laboratory LabCorp found the probability of the DNA belonging to someone else was 1 in 6.8 billion, the lab's forensics technical director Shawn Weiss said.

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Prosecutors seeking the death penalty contend that Williford broke into the Raleigh home of Taft's boyfriend, John Geil, where she had been recovering from surgery and raped and beat the 62-year-old early on March 6, 2010

Defense attorneys challenged Hannon's testimony, bringing up questions raised in 2010 about the crime lab's testing procedures, her qualifications as a DNA analyst and the internal pressure associated with working on a high-profile murder case.

They also tried to keep Weiss from testifying, arguing before Superior Court Judge Paul Gessner that they were at an unfair advantage because they couldn't adequately question the testing procedures because Weiss analyzed only results of tests performed by a LabCorp technician.

Crime scene investigators spent Friday morning showing jurors videos and photos of 2710 Cartier Drive as it was after Taft's attack, as well as Williford's home, less than two blocks away.

At Williford's home, investigators found knives, one of which was hidden beneath his bed mattress, pornographic DVDs and a "journal of sobriety," among other items.

The testimony, which lasted several hours, is standard in criminal trials so that jurors not only get a better sense of the nature of the crime but also the chain of custody and procedures taken to test and protect evidence, even if it is irrelevant to the case.

That evidence included two semen-stained spots on Taft's bed sheet, Rachel Casteline, a forensic serologist for the state crime lab, said.

Police were able to link the crime to Williford, who also faces charges of first-degree burglary and first-degree rape, after obtaining DNA from a cigarette butt he discarded in the parking lot of his apartment building.

Defense attorneys admit that he raped and beat Taft but say his actions weren't premeditated.

They claim that he was incapable of forming the necessary mental intent that is needed for a first-degree murder conviction, because he suffers from a number of disorders and illnesses, including impulse control, sexual disorders and alcohol and drug dependency.

Williford had been doing drugs and was on a quest to get an adrenaline high when he came upon Geil's home, checked the mailbox and saw a large amount of mail in it, attorney Ernest Conner said in his opening statement.

Thinking no one was home, Williford got inside, realized people were there, panicked and started to leave. That's when he noticed Taft's door open, and instead of leaving, went into her room.

Prosecutors, however, contend that Williford knew what he was doing and point to letters that were found in a book at his home in which he talks about choices he made.

"I regret so many decisions that I have made and hope that you understand the evil part of me did get the better of me," he wrote to his wife. "I understand now what is inside me could not be cured."

In another letter to his parents, he wrote:

"You gave your heart and soul into raising me and did everything you could to steer me from the path I was heading down, but for some reason I made these choices. This evil in me I never understood and tried so hard to hide, but some things just can't be changed I guess."

But in the same letter, defense attorneys say, is the proof that Williford is mentally ill.

"'But for some reason, I made these choices,' – choices driven by mental disorders in his brain," attorney Ernest Conner Jr. said.