At Mark Twitchell's first degree murder trial Tuesday, his ex-girlfriend, Traci Higgins, took the stand telling jurors they dated years ago and admitted she had sex with the married man in October of 2008.

Twitchell is charged with first-degree murder in the alleged life-imitates-art murder of Johnny Altinger. Altinger mysteriously vanished on October, 10 of 2008. The Crown alleges after several months after Altinger's disappearance, partial skeletal remains of Altinger were found in a city sewer.

In court a day earlier, a story found on Twitchell's laptop was read out to the jury where the author talks about becoming a serial killer and having sex with a former girlfriend named Laci.

Higgins was asked on the stand about her two tattoos, one of which she said Twitchell helped her design in 2001.

"It's a Celtic cross. I believe it's something that Mark thought would be cool. And it is cool," she said.

In the computer file found on Twitchell's laptop, the author talks about the ex-girlfriend having a Celtic knot-style cross with vines intertwined within it on her left shoulder blade.

During a police interview, Twitchell told a detective about the man who he said sold him the victim's vehicle for $40.

"The best way I can describe it is to call it a Celtic knot."

"What's that?" the detective asked.

"Uh, it's like a circle of interwoven thatch work, I guess you could say. It's kind of yeah, a lot of overlapping pieces that make it look like it's an intricate knotwork designed into a circle," said Twitchell.

Higgins was also shown a receipt for a matinee movie, a horror thriller from the same day Altinger went missing. She testified Twitchell went with her.

And in an interview with police, Twitchell claimed to be cleaning up his film set garage at the same time.

A sheriff also testified Tuesday he gave Twitchell a speeding ticket on the Anthony Henday three hours after Altinger was last seen. In the alleged confession police found on Twitchell's computer, a traffic stop is mentioned, with the author explaining how the ticket was eventually discounted in half.

In court Tuesday morning, the police officer who says he found the deleted files on Twitchell's computer testified he looked at the computer of the alleged murder victim and the computer of a man the Crown alleges went to Twitchell's garage and escaped.

Constable Michael Roszko testified the author wrote about a man named Frank and how Frank escapes from a garage.

Roszko said the opening lines in the deleted file include the statement: "This is the story of my progression into becoming a serial killer." The author writes names have been changed to protect the guilty.

The trial continues Wednesday

With files from David Ewasuk