GREELEY, Colo. -- A woman described in court documents as the "best friend" of Shawna Nelson was arrested Tuesday in connection with the ambush slaying that Nelson is already charged with.

Michelle Dawn Moore, 26, of Windsor, was a defense witness for Nelson's case until she changed her testimony on Nov. 14, according to court affidavits.

The case involves the Jan. 23 slaying of Heather Garraus outside the Greeley credit union where she worked.

Shawna Nelson was arrested a short time after Garraus was shot to death and admitted having an affair and a child with Heather Garraus's husband. Prosecutors said the slaying was the result of the love triangle.

Moore's changed testimony for the prosecution indicated that Nelson told her of her plans to commit the killing, according to court documents.

Moore was charged Tuesday with conspiracy to commit murder and criminal attempt to commit murder, presumably as a result of her admission of prior knowledge of the planned slaying.

According to the affidavit, Moore said that during fall 2006, Nelson began talking about killing Garraus. In early January, Nelson went into detail of the plan -- saying that she would park in the driveway of a vacant house near the credit union, drive her husband's truck so she would not be easily recognized and shoot Garraus in the head when she left work, Moore said.

Moore "told Shawna Nelson she would need to disguise herself. She would need to wear gloves and cover her hair to avoid leaving DNA at the scene," Greeley police detective Greg Tharp said in an arrest affidavit. He had interviewed Moore on Nov. 14 and wrote up the affidavit for Moore's arrest.

Moore also told him that she "cautioned Shawna Nelson to muzzle the gunshot(s) and to pick her shell casings up before leaving the scene ... they spoke about leaving 'misleading' evidence at the scene, which would contain someone else's DNA," he said.

According to the arrest affidavit, Nelson asked Moore to babysit on Jan. 22. Nelson said she was working on a plan but didn't want her to know what it was so that she wouldn't have to answer any questions. Nelson also allegedly told her "I can't deal with it anymore .. If I can't have him, nobody can, she can't ... I just have to take care of it ... I'm miserable. ... I can't live like this anymore."

That afternoon, Moore and Nelson met at a nearby Starbuck's, where Nelson allegedly gave Moore sleep medication and told her to put the crushed Ambien in Nelson husband's Starbuck's coffee, so that he would be asleep during the killing.

"I think what she wanted me to do was to be there with Ken sleeping, to have me be her alibi," Moore told the detective.

But that night, Nelson told her that Garraus "was not where she was supposed to be" so she couldn't executeher plan. Garraus had left work at noon because of a headache.

The two women went out to dinner instead, where Nelson told Moore "I've got to do this ... I've gotta give myself some peace ... I'm going to get rid of Heather tomorrow," Moore said.

The next night, Garraus was killed with a shot to her head.

Nelson allegedly told Moore, "I don't want you to have to say anything. For all you know, I'm going to be taking a bath," according to the affidavit.

Moore admitted that when she received a voicemail message from Nelson that afternoon, saying she was going to take a bath, Moore knew that the slaying would be committed, the deputy wrote.

Moore is being held under a $50,000 bond in the Weld County Jail, where she used to work as a Weld County sheriff's deputy. She was a deputy for four years before quitting in September 2006.

"It is very disappointing that a person that has worked at the Weld County Sheriff's Office would maybe put herself in that position. But she is innocent until proven guilty," said Greeley police spokeswoman Margie Martinez.

The Weld County sheriff has notified his staff that one of their former colleagues is in custody. If Moore is unable to post bond, she may be moved to another jail because she knows the place so well and all of her former co-workers are there.

"It puts the employees in a rather awkward situation," Martinez said.

Moore lives with a Greeley police officer, Timothy Young. He did not come to the door but at family friend told 7NEWS that he is "devastated" by her arrest.

Shawna Nelson's husband, former Weld County sheriff's detective Ken Nelson, is also charged in the case. He was charged as an accomplice in the slaying because prosecutors believe he may have disposed of the gun used to kill Garraus.

Investigators said that Ken Nelson stopped his wife's pickup truck shortly after the shooting and then left. When Shawna Nelson was arrested a few minutes later, they didn't find a weapon.

Heather Garraus was married to Greeley police officer Ignacio Garraus.

Both Ken Nelson and Ignacio Garraus resigned from their jobs and have moved out of state.