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 re-examined and her death was reclassified as a homicide.

Long told jurors that tests conducted on Savio's tissue in 2004 and 2007 found no sign of drugs or alcohol in her system.

The defense has said Savio died from an accidental fall and suggested she have may have been under the influence of some substance.