UNIONTOWN, Pa. -- A 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 yesterday in Fayette County Court, 25-year-old Sarah Sue Hawk, of Uniontown, agreed to testify against her former brother-in-law, Warren Bircher.

Police say 34-year-old Mr. Bircher, of Adah, and Ms. Hawk were having an affair. They say he killed the baby when she gave birth because he thought the baby might be his and he didn't want his wife to find out.

The baby, linked to Ms. Hawk by DNA last year, turned out not to be his. She was found in a backpack in a creek near Uniontown in June 2000 and dubbed Baby Mary by the coroner.

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

Looking for more from the Post-Gazette? Join PG+, our members-only web site. You'll get exclusive sports content, opinion, financial information, discounts from retailers and restaurants, and more. Our introduction to PG+ gives you all the details.