LOWELL - Elena Raucci was there and watched helplessly as a car being driven erratically by an Andover lawyer ran down her husband, killing him in the breakdown lane of Interstate 93.

Yesterday, more than a year after the crash, she was in the courtroom to hear the driver be found not guilty of motor vehicle homicide even though he had admitted being under the influence of the sleep-aid Ambien.

In announcing his verdict, Lowell Superior Court Judge Kenneth Fishman said the defendant, Ki Yong O, 36, a lawyer from Andover, did not know the side effects of the powerful sleeping pills he had taken prior to the accident on the night of June 30, 2006.

"Words can not describe this horrible death and tragedy," Judge Fishman said. "(The situation) is unmercifully enhanced because it occurred in front of the wife and young son."

O was also found not guilty on charges of leaving the scene of property damage.

The judge said O's actions were not voluntary motor vehicle homicide because he did not know the effects of the prescription drug.

O's testimony, where he made conflicting statements to police and had "selective memory" of what happened before the accident, led to "significant doubts to his credibility," Fishman said.

But, Fishman was "unable to conclude (O's) actions were voluntary motor vehicle homicide." The judge called it a "very difficult decision" to make.

During the jury-waived trial, which began in early November, the judge heard testimony from O, Elena Raucci, sleep experts, and witnesses who called 911 the night of the accident. A sleep expert testified that O may have been "sleep driving," a side effect of Ambien that has been reported.

O was seen driving erratically on Interstate 93 when his car slammed into Anthony Raucci, 43, of Methuen. Raucci had pulled into the breakdown lane in Tewksbury to change a tire and was struck while his wife and son looked on. Raucci died instantly.

"For us, this case was always about the victim as well as his wife and young son, both of whom witnessed that absolutely horrific scene" Middlesex District Attorney Gerry Leone said in a statement. "While we must accept today's decision, our thoughts will always remain with the victim's family."

Robert Sheketoff, O's lawyer, called the case one of the most "unfortunate" he's ever been involved in.

"This accident is an unbelievable tragedy," Sheketoff. "This defendant did not do anything criminal. My heart goes out to that family."

O showed no emotion in the courtroom as the judge announced his decision. Outside the courtroom, O hugged a few people and left quickly, surrounded by family and friends. He declined to speak to reporters.

The victim's wife sobbed when the decision was announced and declined to comment.

Sheketoff said soon after the accident, the Food and Drug Administration changed labeling of Ambien to note its side effects.

"Both experts (who testified during the trial) talked about the unfortunate side effects of this drug," Sheketoff said. He declined to comment further.

Constance Rudnick, a lawyer and professor at Massachusetts School of Law, said the prosecution likely did not prove the drug caused the accident, the legal standard for a felony motor vehicle homicide charge.

"The prosecution did not prove that the ingestion of Ambien caused the impaired operation which caused the reckless driving which caused the death," said Rudnick, who is not involved in this case. "There is no question under certain circumstances that Ambien could affect operation. There wasn't proof of that in this case."

The victim's family will likely pursue civil charges, Rudnick said.

"I would proceed with a civil suit," Rudnick said. "It's a different standard of proof. Now you will have to prove he operated negligently. (The not guilty verdict in the criminal case) makes it a little harder, but it is not the end."

The night of the crash, the Raucci family, traveling in separate cars, were returning home from a restaurant in North Reading. Elena Raucci was driving a Saturn, while her husband and son were in a 1991 Geo that got a flat tire. Raucci was trying to fix the flat in the breakdown lane on the northbound side of the highway when he was struck by O's Audi station wagon.

Elena Raucci told police she saw the station wagon coming at her family "at a high rate of speed, flashing its lights on and off." Mrs. Raucci and her son "moved farther out of the way as the vehicle left the roadway, careened onto the grassy area, striking both vehicles and Mr. Raucci," according to a court report prepared by prosecutor Cara Krysil.

While O had just picked up a prescription for 30 Ambien pills, police only recovered 261/2 pills in a bottle on the passenger seat of his car. The instructions said to take one pill at bedtime, according to police.

State Trooper Kevin Baker, who interviewed O after the crash, described him as confused, unsteady and slow to answer questions. O said he did not remember anything after taking the Ambien pill a few hours before the crash.

The sleep aid made headlines earlier in 2006 after U.S. Rep. Patrick Kennedy crashed into a car while he was under the influence of Ambien.