JOLIET, Ill. — Prosecutors at Drew Peterson's trial have begun addressing the central question of whether the former Illinois police officer's third wife was actually murdered.

Forensic toxicologist Christopher Long was the first prosecution witness Tuesday. He told jurors about tests done on tissue samples from the body of Peterson's third wife, Kathleen Savio.

Authorities initially ruled that Savio accidentally drowned in her bathtub. But after Peterson's fourth wife, Stacy Peterson, disappeared in 2007, Savio's body was reexamined and her death was reclassified as a homicide.