FAIRFAX COUNTY, Va. — A Brazilian au pair testified on Wednesday that she turned against her former lover in a sprawling double homicide scheme involving his wife because she “wanted the truth to come out.”

For more than a year, Juliana Peres Magalhães did not speak with officials about the 2023 killings of Christine Banfield and Joseph Ryan, or about Brendan Banfield’s alleged involvement. But days before her own criminal trial, the former au pair changed her mind.

Now, Brendan Banfield is facing a trial in the aggravated murder of his wife and Ryan, and Magalhães' testimony has become a key component of officials' case. Banfield, who has pleaded not guilty, could face life in prison if convicted.

The way prosecutors tell it, Banfield and Magalhães lured Ryan to their house. The two then shot him, staging the scene to look as though Ryan had been predator stabbing Christine Banfield.

“I just couldn’t keep it to myself, the feeling of shame and guilt and sadness,” she said in court of the ruse.

Magalhães is central to that argument. In a Fairfax, Virginia, court, the former au pair testified that she and Banfield had created an account in Christine Banfield’s name on a social media platform for people interested in sexual fetishes. There, Ryan connected with the account, and the users made plans to meet for a sexual encounter involving a knife.

In court this week, she described Banfield’s plan to kill his wife and spend the rest of his days with Magalhães, with whom he had an affair. She testified to the months he had spent plotting their scheme, and the steps he took to manufacture their alibis.

WATCH: Virginian man accused of plotting wife's murder with au pair

John Carroll, Banfield’s attorney, spent much of Wednesday scrutinizing her initial testimony and her motives behind pleading guilty.

He pressed her on who created the social media account and where she and Brendan Banfield were on the day it had been procured. She testified that she did not remember who made the account or what room in the Banfield home they had been in.