“Back then, people said he had been killed by Fidel, others that he had died in Santo Domingo, that he was in Vietnam,” said Juan Carlos Salazar, who, in 1967, was a 21-year-old Bolivian reporter about to begin chasing his first major story. “They placed him here, they placed him over there — but no one knew where he was.”

Loyola Guzmán, a Communist youth leader in La Paz, Bolivia’s capital, would be one of the first to learn. She received a message one day calling her to Camiri, a small town near the border of Paraguay. She said she had no idea what the meeting was for.

Ms. Guzmán is 75 now, but a photo in January 1967 shows her in the flush of youth, in fatigues and a field cap, sitting on a log at a sweltering jungle camp — and next to her is Guevara.

“He said he wanted to create ‘two or three Vietnams,’” said Ms. Guzmán, with Bolivia a base for a revolution not only there but in neighboring Argentina and Peru, as well. Ms. Guzmán agreed with the idea and was sent back to the capital to drum up support for the revolutionaries and manage their money.

In March 1967, the battle began.

Mr. Salazar, the journalist, learned later that month that fighting had broken out between the Bolivian army and an armed group, leaving seven soldiers dead. The reporter was dispatched to the area to investigate, but it remained unclear who the militants were — although it was known they were regularly delivering fatal blows to government forces.

Soon afterward, word began to leak out that the ringleader might be Guevara.

The army wanted to find and defeat him. Among journalists, “everyone wanted to interview him,” recalled Mr. Salazar.