DRAMATIC NEW DEVELOPMENTS IN THE CASE OF A YOUNG MARINE WHO DIED SUDDENLY -- WAS IT NATURAL CAUSES OR POISON? "48 HOURS MYSTERY" ON SATURDAY, FEB.

Marine Sgt. Todd Sommer, 23, seemed to be in peak physical condition before he suddenly collapsed and died in February of 2002. The official cause of death was listed as cardiac arrhythmia, but to one NCIS agent, something about the case seemed a little too familiar.

Prior to that fatal day, Todd had complained of severe stomach pains and nausea, information that led NCIS to order a rarely performed test on some of Todd's tissue samples. The results, prosecutors said, proved that Todd Sommer was the victim of a lethal dose of arsenic. And only one person had both motive and opportunity - his wife, Cynthia.

Although authorities never uncovered direct proof that Cynthia purchased the deadly powder or poisoned her husband, prosecutors believed Cynthia's questionable behavior in the aftermath of Todd's death still gave them plenty of ammunition against her. The military police officer who drove her to the hospital recalled that she never shed a tear, and instead of being in a hurry to get to Todd, she asked if they could stop for cigarettes. Cynthia also called an accountant a few hours after his death and within days was trying to collect the 250 thousand dollars from his life insurance policy. She then partied in Mexico with friends, had sex with several Marines who served with Todd and bought herself breast implants within weeks of her husband's death. Cynthia's actions, prosecutors said, were not those of a typical grieving widow.

In January of 2007, Cynthia Sommer was convicted of Todd's murder. But did she actually poison her husband, or was she being judged for her scandalous behavior? Usually a verdict means the end of a case, but for Cynthia Sommer, the biggest surprise was yet to come.

Correspondent Richard Schlesinger reports the dramatic new developments on 48 HOURS MYSTERY: "An Invisible Enemy," Saturday, Feb. 7 (10:00-11:00 PM, ET/PT) on the CBS Television Network. This broadcast is produced by Marcie Spencer and Gayane Keshishyan. Paul Ryan is the senior producer. Al Briganti is the executive editor. Susan Zirinsky is the executive producer.