BOSTON -- Two summers ago on a pleasant summer evening, Anthony Raucci, his wife and their 7-year-old son were driving home after dinner. A flat tire forced Raucci to pull over into the breakdown lane on Route 93 in Tewksbury. His wife and son were close behind in a separate car.

"I saw this car with its lights going on and off," said Elena Raucci. "He was coming over in the breakdown lane and hit my car. He must have swerved past, hitting Tony's car and Tony. I saw him lying face down way out in the middle of the highway. He no longer had any clothes on and I started screaming."

Anthony Raucci's leg was severed. He was pronounced dead at the scene.

"I'll never forget it," said Raucci. "I see it all the time."

Ki Yong O was behind the wheel of the car that hit Raucci. The pharmaceutical attorney from Andover was charged with operating under the influence and motor vehicle homicide. Blood tests confirmed toxic levels of the sleep drug Ambien. But after a six-day trial in November, a judge acquitted O.

"I was shocked," said Raucci.

In his decision, Judge Kenneth Fishman wrote that "the court is unable to conclude beyond a reasonable doubt that the defendant was voluntarily intoxicated when he operated his motor vehicle."

The defense successfully argued that O was not responsible because he was sleep driving. It's a condition that even sleep experts say is an exceedingly rare side effect of Ambien. The case could set a precedent as the Ambien defense becomes more popular.

"It effects what the brain is capable of doing, and therefore the voluntariness of a person's conduct," said Elliot Weinstein, a defense attorney. "I think it's an available defense for a person who has ingested Ambien."

The Middlesex County District Attorney's office prosecuted the case.

"I think that the defense will be used whenever somebody fits it within the facts of their case. My instincts tell me it will be use inappropriately and in far too many cases," said District Attorney Gerry Leone.

In 2006, Rep. Patrick Kennedy claimed he was sleep driving after taking a prescribed dose of Ambien when he crashed into another car.

Last March in Texas, DUI charges were dropped against a woman who said she was "sleep driving" when she crashed her husband's truck into a neighbor's home.

Last October in Minnesota, a rape case was dropped when the defendant claimed sex was consensual with a female patient in a psychiatric ward. The woman had take Ambien and had vague memories of being assaulted.

But even the defense expert in O's trial tells Team Five Investigates these behaviors occur in one-tenth of one percent of Ambien users. In the case of sleep driving, it's even less than that.

Ambien comes with warnings about side effects. But Don Decker, a drug recognition expert, said drivers are often found to have mixed the sleep drugs with other sedatives.

"People may be taking Ambien, but there are other drugs and alcohol in the system, " said Decker. "People can and do use an Ambien defense as an excuse for their driving under the influence of drugs."

Team Five Investigates obtained results of a recent study by the Academy of Forensic Sciences. The report finds that in seven states that test driver's blood, Ambien is among the top 10 drugs found in impaired drivers.

In the case of O, three and a half tablets were missing from the bottle found in his car at the scene of the accident. And there was conflicting testimony about where and when O took those pills. Elena Raucci worried the acquittal will send a dangerous message.

"It gives people something to hide behind to avoid responsibility for their actions," said Raucci.

The family has filed a civil suit against O. Raucci said she hopes a guilty verdict will give her young son some solace.

"He keeps asking me why did Mr. O take so much medicine?," said Raucci. "Why did he drive if he knew he wasn't supposed to? Why did Daddy die? It's hard enough to come up with those answers for myself, much less to an 8-year-old boy."