The American software millionaire faces deportation to Belize, where he is wanted for questioning in the Nov. 11 shooting death of his neighbor, American businessman Gregory Faull.
ABC News said McAfee, 67, was found unresponsive on the floor of his cell at a detention center in Guatemala City Thursday and was being treated by medics. He had complained of chest pains earlier.
McAfee, an Internet security software pioneer, said he fled to Guatemala, where he was arrested for entering the Central American country illegally Wednesday, to escape persecution in Belize.
A spokesman for Guatemalan President Francisco Cuevas said McAfee's asylum request had no basis and was rejected, clearing the way for his extradition to neighboring Belize, CNN reported.
Before his arrest, McAfee told ABC News Guatemala is a place with "some sanity." He said Guatemalans believe him when he says police in Belize are trying to kill him.
"Instead of going, 'You're crazy,' they go, 'Yeah, of course they are,'" he said. "It's like, finally, I understand people who understand the system here."
Faull, who moved to Belize from Florida, was found dead in his beachfront house on an island in Belize shortly after McAfee's dogs were poisoned. Faull had led a neighborhood petition drive against the dogs, describing them as a dangerous nuisance.
McAfee moved to Belize in 2008, 14 years after he sold his interest in McAfee Associates, the computer security firm he founded in 1987.
The U.S. Embassy in Guatemala City said it would have no comment on specifics of the McAfee situation "due to privacy considerations."