A Brazilian au pair who was having an affair with her employer pleaded guilty to manslaughter on Tuesday and will cooperate with prosecutors when they try her ex-lover on murder charges in the 2023 killings of his wife and another man, the authorities said.

The guilty plea by the au pair, Juliana Peres Magalhães, 24, in Fairfax County Circuit Court in Fairfax, Va., was the latest twist in a case that has drawn widespread attention and generated true-crime podcasts and social media discourse from São Paulo to the suburbs of Washington, D.C.

Ms. Magalhães had been having an affair with her employer, Brendan Banfield, a former agent for the criminal division of the Internal Revenue Service, according to prosecutors. On the morning of Feb. 24, 2023, the police found Mr. Banfield’s wife, Christine Banfield, 37, close to death after being stabbed, and another man, Joseph Ryan, 38, shot dead in the Banfields’ home in Herndon, Va., about 20 miles northwest of the nation’s capital. Ms. Banfield died later at a hospital.

Ms. Magalhães was charged in October 2023 with second-degree murder in the shooting death of Mr. Ryan.

Steve Descano, the attorney for the Fairfax County Commonwealth, told reporters on Tuesday that, as part of a plea deal, Ms. Magalhães pleaded guilty to manslaughter in the shooting death of Mr. Ryan. Her trial had been scheduled to begin in November.

As part of the plea agreement, prosecutors downgraded the murder charge that she faced and agreed to recommend that Ms. Magalhães, who has been in custody, be released with credit for time served after Mr. Banfield’s trial if she continues to cooperate with the authorities. Mr. Banfield is scheduled to be tried in February.