PORTLAND – A police lieutenant charged with operating under the influence pleaded guilty in Cumberland County Superior Court on Tuesday.

Lt. Ted Ross, who headed the detective bureau, had a blood-alcohol content of 0.20 percent when he was involved in an accident after an office Christmas party and a trip to an Old Port bar, according to William Stokes, head of the criminal division of the Attorney General’s Office.

Because Ross’ blood-alcohol level was more than 0.15, he is required to serve a 48-hour sentence, Stokes said.

Ross, an 18-year veteran of the department, will be eligible for an alternative sentence, such as community service, because he has no prior criminal record. He is to report to the Cumberland County Jail on Friday.

Ross was fined $400 and will lose his driver’s license for 90 days.

Outside the courtroom, Ross offered his apologies to friends, family and law enforcement officials.

“It’s been a very difficult situation I’m trying to work through with the support of what I found to be a lot of friends and family,” Ross said.

Police Chief Michael Chitwood said Ross is eligible to keep his job because the offense was a misdemeanor but that his return depends on the results of an internal affairs investigation.

Ross has been on paid administrative leave for the past two months.

Ross was headed home at 10 p.m. Dec. 17 when his unmarked police car struck a pickup truck. Ross told his superiors he was reaching for his cellular telephone when he hit the back of the truck, which was waiting for a parking space.

The impact drove the truck into the Land Rover pulling out of the parking space. The other drivers refused medical attention at the time, but both sought treatment later. They have notified the department they plan to file suit over the accident.

Rescue workers evaluating Ross’ speech, balance and eye response indicated in their report that he responded optimally. Rescue workers and the responding officer said they did not smell alcohol on his breath.

It was not until a search warrant was executed for Ross’ blood tests taken at Maine Medical Center nearly six weeks later that officials learned Ross was drunk, said District Attorney Stephanie Anderson.

Ross had hit his head on his rearview mirror and received a dozen stitches. Anderson said the blood from Ross’ head wound probably masked any smell of alcohol.

The Attorney General’s Office pursued the case after Anderson relinquished her involvement, saying she wanted to avoid concerns that she might not be impartial.