Cbs 48HoursMystery an Invisible Enemy

"We needed to make sure that we had all the answers instead of writing this investigation off," says NCIS Special Agent Mark Ridley. "Twenty-three-year-old young men don't just die."

And it looked like Ridley was dead right.

When the lab tests came back, they showed startling high levels of arsenic in Todd Sommer's liver and kidneys. The levels were high enough to kill anyone, even an apparently healthy 23-year-old Marine.

"The investigation started at that point from being a natural death to really being a homicide," says Ridley.

The NCIS could find no innocent explanation for the high levels of arsenic. Investigators concluded it had to be murder and Todd's wife, Cynthia, had to be the murderess. 

In November 2005, three-and-a-half years after Todd died, Cynthia Sommer was arrested and charged with first-degree murder by San Diego District Attorney Bonnie Dumanis.