(CNN) -- The hunt is on for the killers of two extremely rare whooping cranes, shot as they spent the winter in Kentucky.
The birds are thought to have been illegally shot in a single incident in November, the U.S. Fish and Wildlife Service and the Kentucky Department of Fish & Wildlife Resources said Thursday.
Now a reward is being offered to help track down the killers.
Whooping cranes are the most endangered of all of the world's crane species, according to the release, and are protected under two federal laws.
Fewer than 500 of the long-legged birds live in the wild in the United States. Seven of them were in Kentucky this winter.
The first bird was reported injured on November 25 in Hopkins County, the news release said. Two days later it was rescued because it had become very weak, but its upper leg was shattered and it had to be euthanized.
Investigators then found the carcass of the bird's mate, about 5 miles from where the first crane was discovered.
The birds, which were fitted with radio transmitters, had arrived in Kentucky -- where they'd spent the previous two winters -- only in mid-November.
According to Kentucky's Courier-Journal newspaper, the male of the pair became well known to Louisville-area residents in 2007 when it fell behind during its first tricky migration south.
The reward for information leading to the arrest or conviction of those responsible currently stands at $7,200, and contributions continue to come in, wildlife officials said.
Anyone with clues should get in touch with the U.S. Fish and Wildlife Service or the Kentucky Department of Fish & Wildlife Resources.
The penalty for killing a whooping crane is up to a $100,000 fine and a year in prison.
CNN's Joe Sterling contributed to this report.