The driver who allegedly killed Danbury police officer Donald Hassiak and left the scene pleaded not guilty Tuesday in state Superior Court in Litchfield.

If convicted, he may face a longer prison sentence because he was on probation for another crime when the June 3 accident occurred.

James O'Neill, 47, of Bethel, could spend up to 10 years in prison if he is found guilty of misconduct with a motor vehicle for allegedly running down Officer Hassiak, a New Milford resident, while the 16-year Danbury department veteran bicycled to work on Route 7.

That would mean double the five-year maximum penalty the crime normally would carry.

Litchfield County State's Attorney David Shepack said Tuesday he has filed a notice requesting the harsher penalty in the event of conviction.

The so-called 53a-40b notice, named for the state statute that permits the practice, leaves the decision about the stiffer sentence up to the judge.

"It's a discretionary enhancement," he said.

Mr. O'Neill, who is represented by public defender Christopher Cosgrove, also is charged with evading responsibility, driving without insurance, and tampering with evidence in connection with the fatality, which occurred about 10:30 p.m. on June 3.

He pleaded not guilty to all charges during an appearance Tuesday in state Superior Court in Litchfield.

Both the misconduct and tampering counts are Class D felonies, but the notice filed by Mr. Shepack asks the court to impose sentences consistent with the more serious Class C felonies.

Investigators connected Mr. O'Neill to Officer Hassiak's death after witnesses recognized his gray Buick Skylark as the vehicle they saw speeding south on Route 7, away from the scene.

When police later located the Buick, they found damage consistent with the accident, including pieces of Officer Hassiak's clothing embedded in the broken windshield.

At the time of the accident, Mr. O'Neill, who has a long record of motor vehicle and criminal convictions, was serving two years of probation as a result of a 2007 conviction in state Superior Court in Danbury for interfering with an officer and breach of peace.

Mr. O'Neill, who also faces evading responsibility and other charges for an accident in Bethel that occurred in May, is serving seven months in prison for violating his probation in the 2007 case.

He is due back in court Sept. 14.