Mary Costello is the former office manager of Dr. Barragan who said she had supplied the murder victim Maggie Schilling with drugs. She and her husband are the murderers of Maggie Schilling. Together, they kidnapped Maggie Schilling and held her for a ransom of $1 million. During her captivity, Schilling's feet and hands were bound, and struggling against the binds caused her bones to break. To keep Schilling quiet, the Costello's gave Schilling hospital heroin and caused her to overdose.