FORT COLLINS, Colo. -- Shawna Nelson sat quietly as a prosecutor told jurors Thursday that she killed a former lover's wife so she and her lover could live together with their son.

Defense attorneys painted a different picture of Nelson and argued she was over her obsession with her lover and did not kill Heather Garraus, 37.

The first to take the stand were co-workers who witnessed the shooting outside the credit union in Greeley where Garraus worked.

Brad Mitchell was one of the first to call 911. Mitchell sat with his head down as the tape was played back.

MITCHELL: "Somebody yelled at her, 'you ruined my life,' in a mask and just shot her and ran."

DISPATCHER: "Where did they go? Are they on foot?"

MITCHELL: "They're on foot and they're heading east."

Nicole Cordova, who also was in the parking lot that night, covered her eyes as tears streamed down her face.

DISPATCHER: "Are you with the person that was shot?"

CORDOVA: "(scream) she's on the ground. (scream)"

DISPATCHER: "...male or female?"

CORDOVA: "Female. Heather Garraus. Her husband's a cop."

Nelson, 36, is charged with first-degree murder in the shooting of Garraus on Jan. 23, 2007. If convicted Nelson faces a life in prison term without the possibility of parole.

Nelson's family has said Nelson, a former Weld County sheriff's dispatcher, had an affair with Garraus' husband, former Greeley police officer Ignacio Garraus, and that the affair produced a son. Nelson also was married at the time.

Prosecutor Greg Lammons said in opening arguments Thursday that Nelson wanted Heather Garraus gone so that she, her son and Ignacio Garraus could live together.

Defense attorney Annette Kundali said Nelson was not the killer and that police refused to consider other suspects, such as loan applicants Heather Garraus rejected.

"We're not going to be able to tell you who did kill Heather Garraus we don't know, but it wasn't Shawna Nelson," Lundelius said.

The trial, being held in Fort Collins is expected to last more than two weeks.