One of two eggs laid by a bald eagle at a central Illinois zoo is missing and authorities think the culprit could be a raccoon or a human.

The discovery was made Friday afternoon at Miller Park Zoo in Bloomington, where attendance has been up since an eagle named Beauty laid eggs for the first time in her 13 years at the zoo.

No broken egg shells were found, said zoo director John Tobias, and both eggs appeared intact on Thursday.

Beauty laid the eggs during a brief visit from a wild eagle, presumed to be a male, that perched in trees over the enclosure for four days in late April. If the remaining egg is fertile, it could hatch around Memorial Day.

A captive male eagle named Mathata has been helping incubate the eggs. Neither eagle can fly because of injuries they received in separate accidents that led to their captivity.

Raccoons live nearby, Tobias said, but one would have to be brave to venture into the enclosure and face two protective birds.

His only other theory is that a human stole the egg. The enclosure's perimeter fence is 8 feet high.

"I'm not saying someone came in and stole it," he said. "I'd like to think no one did, but there were no signs of a raccoon in that exhibit before."