MATTAMISCONTIS TOWNSHIP, — An unidentified man armed with a rifle is dead after spending more than four hours Monday threatening to shoot himself in a rented U-Haul truck stopped on Interstate 95 near Lincoln, state police said.

The man died shortly after 8 p.m. Police declined to identify him, pending the notification of the man’s relatives, or the woman who was driving. Investigators would not say how he died, but police “were not involved in the man’s death,” state police Lt. Christopher Coleman said.

“We certainly suspect this is a self-inflicted gunshot wound,” said Stephen McCausland, spokesman for the Maine Department of Public Safety.

The Maine State Medical Examiner’s Office will determine the cause and manner of death, Coleman said. ME’s office workers were headed to the truck, where the man’s body remained, late Monday.

State police negotiated with the man for four hours.

“We’re still trying to get our arms wrapped around this incident and investigation,” McCausland said Monday. “I suspect there will be more information released tomorrow as our investigation continues through the night and into tomorrow.”

The man and woman, whom Coleman referred to as a couple, had left a motel in Medway shortly before Penobscot County Sheriff’s Department deputies pulled the southbound truck over near mile marker 225 at about 4:05 p.m. The man let the woman leave the vehicle after police surrounded it, state police said.

The man was holding a gun to his head, McCausland said. It was unclear whether the man threatened himself with the .30-30 rifle that state police said he had. The traffic stop occurred as part of an ongoing investigation, but Coleman and state police Lt. Wes Hussey declined to be more specific.

“There was a woman inside who left the truck and has now been questioned by deputies and state police. She has not been charged with anything,” McCausland said.

Coleman and Hussey declined to say whether the man threatened the woman.

A Lincoln resident who was heading south to Bangor for a work meeting said he saw the U-Haul truck apparently a few minutes after it had been pulled over.

“It looked like an accident where police were blocking the road and there was traffic stopped in the right-hand lane,” said 47-year-old Paul Nantkes, a safety director for H.C. Haynes Inc. of Winn. “We were probably 250 to 300 yards from where the police were.”

To Nantkes, it appeared that state police had stopped the truck. He said he saw four or five light blue and black police cruisers around the truck and blocking the highway. Two state police cruisers also were stationed farther south, both blocking the southbound lanes, he said.

“It is possible that the sheriffs got there first,” he said.

Police closed both the north- and southbound lanes of I-95 between the Howland and Lincoln exits at 4:35 p.m. Traffic was detoured to Route 116 and Route 2, McCausland said.

Rumors flew among the several motorists who stood on the road amid their stopped vehicles until a sheriff’s deputy approached them about 30 minutes into the incident, Nantkes said.

The deputy explained that there was “a gentleman in the U-Haul with a rifle and that he [the deputy] wanted to get us out of there in case there was any shooting,” Nantkes said.

Nantkes found that disconcerting — “Initially, I thought of my children,” he said — but not as unsettling as the rumors.

“They were saying it was everything from a bomb to drugs to a kidnapping,” Nantkes said.

Nantkes found it almost as jarring, he said, to get away from danger by driving northbound in the southbound lane to a median crossing before exiting at exit 227, he said.

“It took some getting used to,” Nantkes said.

Police reopened the northbound lane of I-95 at 7:45 p.m. Both lanes were open at about 9:25 p.m.

State police detectives from the Major Crime Squad expected to be working the case overnight into Tuesday, said Coleman, who praised Penobscot County sheriff’s deputies for stopping the vehicle.

“We are thankful that they stopped the truck and no one was injured, other than the man himself,” Coleman said during a press conference near exit 227.

East Millinocket and Lincoln police were among several local departments that assisted state police and county sheriffs with the investigation.

“The police were very professional. They had control of the situation,” Nantkes said. “They had plenty of manpower there.”