Sheriff Jerry Demings said Johnson appeared to have met Byrns in a video game chat room and that he posed as a 13-14 year old.
Police were first notified about a missing child Sunday morning, after the girl's parents say she disappeared overnight.
"This is someone that she's been communicating with online under the guise of being a female and basically has just pulled her into a trap, so to speak", her father Steve Johnson told the broadcaster.
Her parents were aware of the communication but the man claimed to be a young woman who lived in IL.
Police said Byrns drove from IL, picked up Johnson down the street from her Florida house and attempted to bring her back to his home.
Demings said the episode was a "cautionary tale" that should remind parents to monitor their children's online activity and be aware of who they are communicating with.
On Sunday, Alice Johnson's dad noticed she was gone when he tried to wake up her for church. He credited "extraordinary police work" for "the best possible outcome any of us could have hoped for".
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The 11-year-old, Alice Amelia Johnson, was found in Georgia after vanishing from her Orlando home on Saturday night.
Byrns is now in custody charged with kidnapping.
Police are saying that Johnson is doing okay, from a physical standpoint.
"I would say she's lucky to be alive".
"In this case, this individual on the other end (was an) adult male who is a child sexual predator and that's all of our worst nightmares", police said.
Bryns is in custody and more charges may be filed.