GRAND LAKE, Colo. -- A 92-year-old man who was attacked by a bull moose while walking to church in a small mountain town was upgraded to serious condition on Monday.

Authorities said the adult moose attacked Louis Heckert on Sunday in Grand Lake, a resort village of about 500 year-round residents 50 miles northwest of Denver on the western edge of Rocky Mountain National Park.

The nature of Heckert's injuries was not disclosed. He was taken to St. Anthony Central Hospital in Denver, where he was first listed in critical condition.

Heckert is a former mayor of Grand Lake and had just celebrated his birthday on Saturday.

The moose was shot and killed by a state Division of Wildlife officer, agency spokesman Randy Hampton said.

"All indications are that the moose attack was unprovoked," said DOW Regional Manager Ron Velarde. "The DOW will not tolerate wildlife aggression towards people and in this case we felt fully justified in killing the moose."

The moose had serious injuries from some previous accident, which could have contributed to his behavior.

"We suspect it probably did but there's just no way to know," Hampton said.

The moose had five broken ribs on his right side -- one which punctured its chest cavity -- and several separated vertebrae, Hampton said. He said officers did not know what caused the injuries but it could have been a collision with a vehicle. An examination of the carcass showed that the injuries were older and had begun to heal.

The animal appeared to be 5 or 6 years old. He was not weighed, but Hampton said a bull moose of that age could weigh about 800 pounds.

Moose can be aggressive, but such attacks are rare, Hampton said.

"They're big animals. They're not afraid of people," he said.

Aggressive behavior toward humans is more common when cow moose are protecting their young in the summer or during the mating season in late summer or fall, he said.

"The significant pain the moose was in may have contributed to the attack. However, officials concede they will never know what lead to the attack. Additional veterinary tests are planned," said Velarde.

A wildlife officer had also responded to an incident involving a moose two weeks earlier in the same area. In that incident a woman was knocked to the ground and stepped on by a charging moose that may have been startled by her dog, which was not on a leash. The woman was treated and released for her injuries. Officers say there is no way to be sure if the incident involved the same moose.