By ANDREW PAPARELLA, JASMINE BROWN and ALEXA VALIENTE

Working undercover on his first case as a fake hit man, an officer said he was more nervous about messing up the case than he was about the danger posed by his secret identity.

"I don't want to say the wrong things, just you know, mess up the case or anything like that," Jose, whose name has been changed because he still works undercover, told ABC News' "20/20."

Jose is part of an elite squad of undercover agents working for the Bureau of Alcohol, Tobacco and Firearms all across this country. Earlier this year, a group of undercover federal agents agreed to sit down with "20/20? to recount the stories of intervening in murder-for-hire plots.

The setting for Jose's first undercover case was a supermarket parking lot in 2011. There, in the midst of grocery shoppers, he met with soccer mom Nicole Faccenda, 42, of Lyndhurst, New Jersey.

"I guess she wanted to make it seem like she was shopping, if anyone questioned her, what she was doing here at the time," said Jose.

Faccenda wanted the new girlfriend of her longtime boyfriend, Howie, killed, authorities said. She hired Jose, whom she met through Howie's relative, to get the job done.

"Just based on looking at her, you didn't really think that she was capable of doing it," Jose said. While Faccenda plotted the murder of her romantic rival, she wanted Howie to live - but not because she was carrying a torch for him.

"She wants him to live because she wants him to grieve," Jose said. "She wants to see him suffer."

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Jose said he worked out the details of the plot with Faccenda. He asked her how she wanted the murder carried out and how much she was willing to pay. They agreed on the price of $10,000. Faccenda later paid a down payment of $2,000 for the hit, authorities said.

Faccenda was arrested after the week-long ATF operation. She later pleaded guilty and, in December 2013, was sentenced to 10 years in prison.

"I thank God every day that no one got hurt from my careless actions," Faccenda told "20/20? in an email from prison. "I would like to apologize to the victim and her family. I am not - and have never been - a violent person. I was a woman destroyed by emotions and I am paying for it dearly."

Jose recalled thinking, "What's wrong with you?"

"When someone's being so evil next to you, it's like, 'How can you even conceive of something like this?'" Jose said.

"And you have to sit there with your poker face, and, 'Okay, if this is what you want done, this is what I'm going to do.'"

Watch the full story on ABC News' "20/20? on Friday, May 30, at 10 p.m. ET