That dire view was reinforced last week when Cuba's foreign minister backed away from his prediction that the ailing Castro would return to power by early December. "It's a subject on which I don't want to speculate," Felipe Perez Roque told The Associated Press in Havana.
U.S. government officials say there is still some mystery about Castro's diagnosis, his treatment and how he is responding. But these officials believe that the 80-year-old has terminal cancer of the stomach, colon or pancreas.
He was seen weakened and thinner in official state photos released late last month, and it is considered unlikely that he will return to power or survive through the end of next year, said the U.S. government and defense officials. They spoke on condition of anonymity because they were not authorized to speak publicly about the politically sensitive topic.
With chemotherapy, Castro may live up to 18 months, said the defense official. Without it, expected survival would drop to three months to eight months.
American officials will not talk publicly about how they glean clues to Castro's health. But U.S. spy agencies include physicians who study pictures, video, public statements and other information coming out of Cuba.
The CIA's Office of Medical Services, for example, studies hair and other biological samples for hints about world leaders' health and how that could affect their official duties.
Images and video of a weakened Castro released in late October showed his now-slight frame and shaky movements. They contradicted the athletic image he sought to portray in his red, white and blue Cuban Olympic team warm-up suit, emblazoned with "F. Castro" on the chest.
A dark lesion on his neck could be seen in some images and a baggy nylon jacket could be hiding a colostomy bag. But the photos also made clear that he has not lost his hair or beard to chemotherapy.
Cuba has only known one leader in 47 years. Castro temporarily ceded power to his brother, Raul, at the end of July just before the government announced that the president was having intestinal surgery.
A planned celebration of Castro's 80th birthday next month is expected to draw international attention. The Cuban leader had planned to attend the public event, which already had been postponed once from his Aug. 13 birthday.
Perez Roque, the foreign minister, said last week that Castro was recovering steadily from his intestinal surgery. "We are optimistic," he said.
But the minister also said there was no guarantee Castro would be well enough to attend the birthday celebration.
Brian Latell, a former Latin American specialist with the CIA who has written a book examining the leadership of Fidel and Raul Castro, said he has been convinced for three months that Castro is gravely ill with inoperable cancer.
Questions abound about what comes after Castro.