FORT COLLINS, Colo. -- Opening arguments are expected Thursday afternoon in the Shawna Nelson murder case.

She's a former police dispatcher accused of shooting and killing Heather Garraus in a Greeley parking lot early last year.

Nelson had an on-again, off-again relationship with Garraus' husband, Ignacio, and gave birth to a son conceived during that affair. Ignacio was a Greeley police officer at the time.

At the beginning of jury questioning Wednesday afternoon, prosecutor Clifford Riedel told potential jurors that, "Nelson viewed Heather as an obstacle." He said Nelson wanted Ignacio to leave Heather and to start a new family with her.

But Ignacio wouldn't leave his wife.

Reidel told jurors that Shawna Nelson confided to a friend that she was going to kill Heather Garraus.

"On Jan. 23, 2007, a person dressed in a black gown and grim reaper mask shot and killed Heather," he said. "That person was Shawna Nelson."

But public defender Keven Strobel said Nelson denies that.

He said Shawna Nelson was upset that Ignacio Garraus wanted to terminate his parental rights with their son, but added that she was reuniting with her husband, Ken Nelson, who at the time was a deputy with the Weld County Sheriff's Office.

Strobel said, "On that night (Jan. 23, 2007) Shawna Nelson was on the other side of Greeley driving to a liquor store."

He said Nelson is not and could not be the person who killed Heather Garraus.

More than 130 prospective jurors were summoned to the Larimer County Justice Center for this case.

Attorneys are trying to whittle that number down to 38.

From that group, they will select 12 jurors and two alternates who they think can be fair to both sides.

Although the shooting happened in Greeley, which is in Weld County, the trial is being held in Larimer County, because the suspect's husband had a relationship with an employee in the Weld County District Attorney's Office.

Ken Nelson has since resigned from the Weld County Sheriff's Office, and Ignacio Garraus has resigned from the Greeley Police Department.