UNIONTOWN, Pa. -- A Fayette County woman has pleaded guilty to third-degree murder for her role in the death of her newborn baby girl more than nine years ago.

In exchange for her plea Thursday, Sarah Sue Hawk, 25, of Uniontown, agreed to testify against her former brother-in-law, Warren Bircher.

Police said Bircher, 34, of Adah, and Hawk were having an affair when Hawk was then 16. They said he killed the baby when she gave birth because he thought it might be his and he didn't want his wife to find out.

The baby, linked to Hawk by DNA last year, turned out not to be his. Police said the infant's body was discovered on June 4, 2000, by a fisherman who noticed a bag floating down Cove Run Creek near Uniontown. The baby was dubbed Baby Mary by the coroner.

Hawk faces up to 20 to 40 years in prison when she's sentenced in January.