The carcass of the big grizzly, called Maximus because he stood 7 1/2 feet tall and weighed about 800 pounds, was found Aug. 12 on a ranch in northern Montana. The bear had been dead about a month.
Special Agent Brian Lakes said Friday that he didn't know what the exact size of the reward would be but it would be "substantial."
"We are following up on some leads at this time," Lakes told the Great Falls Tribune.
In 2007, the 9 1/2-year-old bear, officially identified by authorities as No. 4273, was captured accidentally by the Montana Department of Fish, Wildlife & Parks. At the time, the department had set out to catch females for a study.
Maximus then weighed 765 pounds and was the second largest bear managers had ever captured in the state, said Mike Madel, a grizzly bear management specialist with the department.
A remote camera snapped a picture of the same bear in the fall of 2008, and Madel said it looked as if the bear had grown to more than 800 pounds. He said the bear likely weighed more than 800 pounds at the time it was killed.
Typically, he said, male bears in the region average around 600 pounds.
The bear that was killed didn't have a history of feeding on livestock or having run-ins with people, said Madel, and was shot outside of black bear hunting season. Hunters sometimes mistake grizzlies for black bears.
Lakes said he is also investigating two other illegal shootings of grizzly bears.