The northwoods black bear that attacked a pet dog and three men last month near Isabella was deranged by severe brain swelling and had two external abnormalities that suggest the animal possibly was captive at one time.

Minnesota Department of Natural Resources bear research biologist Dave Garshelis said in an exclusive interview Friday that the agency is still investigating the bear’s history and is asking the public to call the DNR’s information hot line with any tips. Meanwhile, veterinary pathologists at the University of Minnesota continue to probe what caused the bear’s encephalitis. Bacterial causes have been ruled out, Garshelis said.

“They characterized the swelling as severe, extensive and subchronic [reoccurring],’’ Garshelis said. “It would have significantly influenced the bear’s behavior … even for several weeks before the attacks.”

Garshelis, based in Grand Rapids, was among the first to examine the bear after it was fatally shot in the face by a DNR conservation officer from Ely and a Lake County deputy sheriff. The officers were on an emergency assignment to stop the bear’s rare rampage. The animal’s life-threatening attacks occurred in daylight on Dec. 19 outside two residences deep in the woods near McDougal Lake.

Garshelis said pathologists were able to delineate the profound swelling in the animal’s brain from the acute injury from the blast. He said the case involves two other mysteries: a swath of missing hair all around the bear’s neck and damaged rear claws.

On the neck, scar tissue was built up and hair follicles were missing in a complete ring ranging from 1 to 2 inches wide, Garshelis said. He said the distinct injury makes him think the bear had a snare or collar around its neck at one time.

But unlike research collars that sometimes cause temporary hair loss in certain patches, the missing hair on the diseased bear was permanent and fully circular, he said. “It was something around the bear’s neck that either fell off or was taken off and the hair didn’t regrow,’’ Garshelis said. “That part of it is a little difficult to understand.”

With bacteria ruled out as a cause for the bear’s brain inflammation, officials don’t see how the neck trauma could be related to the encephalitis. Garshelis said pathologists are checking possible viral or fungal causes for the swelling.

If the brain problem is linked to a virus, Garshelis said, he will wonder if it was transmitted by other animals in a confined space. If the bear was being kept illegally as a pet, he’s hoping someone in the public will come forward with information. The DNR TIPS line is 800-652-9093.

When people take in wild bears in violation of game laws, they’ve been known to trim the front claws to avoid injury. In this case, the bear’s back claws appear to have been clipped by a person or damaged from a hard surface. It’s a puzzling condition, Garshelis said, especially because the officers who tracked the bear said it walked normally and didn’t drag its feet.

“Those claws are a mystery,’’ Garshelis said.

The male bear was just shy of its third birthday and carried a normal weight of just under 140 pounds. Investigators retraced the bear’s last steps and found that it had been hanging around three unoccupied cabins in the woods near Isabella before attacking Bill Vagts’ small pet dog.

Vagts wrestled the bear off the dog and was bitten in his stomach. The dog also was bitten, and bruised. From there, the rogue bear encountered a pair of construction workers who were building a garage at a neighbor’s house.

Garshelis said the bear stood up and grabbed one of the men by the chest, piercing his clothing. As they wrestled, the other worker “clobbered’’ the bear with a leveling tool. The bear turned on his attacker and bit him in the leg. The first victim then picked up a shovel and pounded the bear in the head, giving both men time to retreat to a parked van.

Garshelis said the bear was oddly passive when the two officers found him a short time later in the same vicinity. The bear was curled up in the snow next to a fallen tree and merely looked up when the officers arrived. A bear in its right mind would have run from the officers, he said.

Officials retraced the bear’s footprints in hopes of finding where he lived. They lost the trail amid a mass of wolf tracks more than a mile away from the first attack.

“We don’t know if this bear was denned or not,’’ Grashelis said.

He said it was fortunate for other people that the bear was destroyed after the second attack.

“I have a feeling in this case the attacks would have continued,’’ Garshelis said. “This bear was really messed up.”