Kaleb Langdale had been swimming in the Caloosahatchee River in Moore Haven Monday when the alligator attacked, CBS News reported.
Jorge Pino of the Florida Fish and Wildlife Conservation Commission said the alligator was found and killed Monday evening, and officials said the part of the boy's arm was found inside the alligator, WBBH-TV, Fort Myers, reported.
But friends of Langdale said Monday night doctors were unable to reattach it.
Langdale was flown to Lee Memorial Hospital in Fort Myers after the attack.
Matthew Baker, who had been in the water with Langdale, said the alligator was about 10 1/2 feet long.
"[Langdale has] been around alligators all his life. He's smart enough. He knows if he offers him his arm, he won't take his torso. He was smart, he took the risk," Baker said.
FWC officials said it's alligator mating season when attacks are not uncommon.
"Any type of splash in the water is prey for them," FWC Capt. Jeff Ardelean said.