The boy was swimming with friends Sunday when he disappeared, the Australian Broadcasting Corp. reported.
Another boy, also 12, managed to fight off a crocodile estimated at 8 to 10 feet long before it grabbed his friend, authorities said. Territorial police killed two large crocodiles, 14 and 15 feet in length, Monday morning.
"Both of those were shot and removed from the area," Northern Territory police Sgt. Stephen Constable said. "We've since had a look at both crocodiles and neither of them had anything in their stomachs, and we're going to continue the search today."
Ranger Garry Lindner said the two larger reptiles were killed because they could have moved in after the smaller crocodile grabbed the boy. "... You've got to take all of that into account when you're dealing with a situation," he said.