The Los Angeles Times has a feature-length story titled An American adventurer's death in El Salvador. The article tells the story of Joe Sanderson, who joined the FMLN guerrilla movement during El Salvador's civil war:
Joe Sanderson is one of two Americans known to have fought and died with the guerrillas of the Farabundo Marti National Liberation Front, the leftist rebels whose war against El Salvador's U.S.-backed military junta was one of the last conflicts of the Cold War.
Rescued from the battlefield by a rebel historian, Sanderson's 330-page diary and other writings lay neglected and unread for decades. The guerrilla veteran who saved the diary recently allowed me access to it, the first time an outsider had seen it.
The diary and the hundreds of missives Sanderson wrote home tell an unlikely American adventure story. They chronicle a peripatetic Midwesterner who joked and charmed his way across five continents, and eventually fought against an army backed by his own government. (more)
The guerrilla veteran who saved the diary, was Carlos Consalvi, whose nom de guerre was "Santiago," and who ran the FMLN's clandestine radio station, Radio Venceremos. Today Consalvi is the man behind the Museum of Word and Image in San Salvador.
An academic paper by Kristin Cheasty Miller included in the 2008 ILASSA Student Conference Proceedings, titled Frente Sandalista: the Relationship between U.S. Activists and the Revolutionary Vanguard in El Salvador, 1979-1989 looks at other US activists on the side of the FMLN during the war years.