Fire officials told The Associated Press they were blocked from entering the prison for half an hour by guards who thought they had a riot or breakout on their hands.
Relatives are suspicious because Honduras has been the site of two other major prison fires, in 2003 and 2004, that killed a total of 176 inmates. Government officials were convicted of wrongdoing in the 2003 blaze, which investigators blamed on the guards.
Prison officials and the governor of Comayagua province originally said the fire was started by an inmate who screamed he was going to burn the place down in a cell phone call to the governor, Paola Castro.
On Thursday Castro said the call was actually a message from someone reporting the fire and that she accidentally erased it.
Officials then said prisoners told investigators that the fire started with a fight inside a prison barracks over a mattress.
One prisoner threatened to burn the mattress if the other didn't hand it over, said Elver Madrid, director of intelligence for Honduran national police. Madrid said his office currently considered that to be the most credible scenario.
"They're assassins," said Pedro Mejia, a weather-beaten, straw-hatted farmer whose son, Carlos David Mejia, died in Comayagua. His son had been there for six months while awaiting trial for the attempted robbery of a welding machine.
The U.S. State Department has criticized the Honduran government for harsh prison conditions, citing severe overcrowding, malnutrition, and lack of adequate sanitation.
The Inter American Court on Humans Rights issued a report in 2006 recommending measures to avoid prison overcrowding and training and equipment to deal with emergencies and evacuations after the fires in 2003 and 2004. It issued another critical report in 2010 noting that none of the changes had been made.
The State Department said it was sending Bureau of Alcohol, Tobacco, Firearms and Explosives investigators to Honduras. The team will include forensic chemists, explosives enforcement officers and dogs that can sniff out explosives and accelerants.
Howard Berman, then-chairman of the U.S. House Committee on Foreign Affairs, questioned U.S. aid to Honduras last fall, saying human rights abuses involving security forces had "reached a distressing pitch."
"The most chilling aspect of this rather gruesome set of problems is that U.S. government assistance is flowing into the thick of it," Berman wrote to Secretary of State Hillary Rodham Clinton.
Mendoza reported from Mexico City. Associated Press writers Alberto Arce from Mexico City and Christine Armario in Comayagua, Honduras, contributed to this report.
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