PITTSFIELD – A Portland man was arrested Tuesday night, just 24 hours after he allegedly shot a Portland taxi driver in Pittsfield.

Derek Wilhelmsen, 22, was charged with murder at the Portland Police Department, where he had been interrogated most of the afternoon.

After his arrest, Wilhelmsen was taken to Somerset County Jail and was expected to make his first court appearance today or Thursday. State police spokesman Stephen McCausland said police were still trying to determine a motive.

The homicide was Maine’s first this year, and the first in Pittsfield in nearly seven years.

The dead man was identified late Tuesday as Nunzi Mancini, 38.

The body was discovered at 3:40 a.m. behind the wheel of a late-model, gray Mercury Grand Marquis just off Route 100 in Pittsfield.

On the roof of the car was a white “taxi” sign, and a magnetic sign in red and white advertised “Tony’s Taxi” on the front doors. The driver was dead, killed by a gunshot wound to the head.

McCausland said that the vehicle was owned by a Westbrook man, Tony Pasquali, and that the driver was either leasing it or in the process of purchasing it.

When Pittsfield police Officer Nicholas Fletcher found the taxi parked at the top of a driveway off Route 100 early Tuesday, he initially thought he had stumbled upon a suicide scene.

Within 15 minutes, however, Fletcher had determined that the driver of the Westbrook-based cab had met a suspicious death.

“We are treating this as a homicide,” Pittsfield Police Chief Steven Emery said later Tuesday morning.

By early afternoon, Wilhelmsen had been taken into custody by Portland police and was being questioned by detectives. McCausland confirmed that Wilhelmsen was linked early Tuesday to the shooting when a former girlfriend who lives in Pittsfield came forward to say she had given Wilhelmsen a ride from the Route 100 area to Bangor Monday night. With them were the girlfriend’s young child and a female friend. McCausland would not identify the woman. He said the four drove around Pittsfield for a time before heading for Bangor.

McCausland said that Wilhelmsen spent Monday night at the Howard Johnson’s Hotel on Odlin Road in Bangor, and had checked out about 6:45 a.m.

“He was a Concord Trailways bus employee and he took a bus back to Portland,” said McCausland.

Around noon, Portland police received a telephone call from Ingraham Volunteers, a suicide hot line, and went to a location where they took Wilhelmsen into custody. McCausland said Wilhelmsen was taken to an area hospital and returned to the Portland Police Department for questioning.

Meanwhile, the taxi with the body still inside was taken Tuesday afternoon to the Maine State Police Crime Laboratory in Augusta. The body then was taken to the State Medical Examiner’s Office for an autopsy.

The taxi was found by Fletcher about 3:40 a.m. less than six feet from the road, sideways in front of a roadside residential garage at a property owned by Cara Lee Page. Page was formerly active in town affairs and was a member of the Pittsfield Planning Board. She now lives out of state.

The car appeared to have been driven into the driveway from a northbound travel direction, headed into Pittsfield from Burnham. The engine was not running but the interior light was on, said McCausland.

Although McCausland would not estimate the time of the shooting, a witness reported driving by the area and seeing a man walking away from the vehicle between 9 and 10 p.m. Monday.

Shortly after finding the body, Fletcher reported over his radio from the scene that he had observed footprints leading from the taxi to the home.

The yellow wood-frame home sits alongside the Sebasticook River down a long drive, at least 400 feet from the road. It is listed for sale with Ames Whittemore Realty. Realtor Dwayne Ames confirmed Tuesday that the home was being rented. Ames declined to say who was renting the home.

After he discovered the body, Fletcher was joined by Pittsfield Police Chief Emery and Somerset County sheriff’s Deputy Sgt. Pierre Boucher. The Maine State Police Criminal Investigation Division was notified, and investigators immediately headed for Pittsfield. A Sebasticook Valley Hospital Ambulance waited down the road but was not needed.

Later Tuesday morning, the state police mobile crime lab was parked on Route 100 behind the taxi. The vehicle had been cordoned off with yellow crime scene tape placed atop snowbanks. The front passenger window of the taxi was shattered and the door was hanging open. Detectives swarmed the area as light snow began to fall.

McCausland would not confirm whether shell casings were found on the ground outside the taxi. He said a search warrant head been obtained for Wilhelmsen’s Portland apartment at 50 Spruce St. Officers were searching for a 22-caliber handgun believed to be the murder weapon.

Although Wilhelmsen is from Portland, McCausland said officials did not believe the Pittsfield killing was connected to an early morning shooting in Portland.

Portland police shot and killed a man suspected of a Gardiner armed robbery as the man left a taxi with a gun about 3 a.m.

“Certainly, we have been in contact with the Portland police,” said McCausland, “but at this point there is no known connection between the two incidents.”

The last murder in Pittsfield took place in July 1994 when John O’Palenick, 35, killed his 6-year-old daughter, Georgeanne, and then took his own life. O’Palenick had recently divorced from his wife and committed the murder-suicide just days after taking his daughter for a month’s summer vacation.

The bodies were discovered by the child’s mother, Trudy O’Palenick, after she spotted her ex-husband’s car parked at the foot of the driveway where she was living.