As workers cleaned up the rubble of the century's deadliest prison fire, a collective rage built among relatives who gathered at the morgue and said the official explanation of a mattress fire was absurd.

Details of the investigation remained thin, and mystery swirled around the possible cause, from a crazed inmate who set fire to his bedding to rumors that gas cans were found inside and that guards deliberately set the blaze, whose death toll rose to 356 when a 31-year-old prisoner died in a hospital early Friday.

Family members said guards fired on prisoners to keep them from fleeing the flames, though guards and firefighters said they were shots in the air to summon help and to respond to what they thought was a prison break.

The attorney general's office said it was investigating all angles.

"It's impossible to believe that prisoners set the fire themselves when they too were going to die," said Felix Armando Cardona, 56, whose son, Luis Armando Cardona, 28, died in the blaze that broke out in Comayagua prison late Tuesday night.

In Geneva, the U.N.'s human rights office said Friday that an independent probe is needed and that Honduras must prevent a recurrence since it was the third fatal prison fire in a Honduran prison in a decade.

From the time firefighters received a call at 10:59 p.m., what should have been a rescue became a catastrophe.

Only six guards were on duty, four in towers overlooking the prison and two overseeing 852 people crowded into a facility built for half that number. Some 57 percent had yet to be convicted, either awaiting trial or being held as suspected gang members, according to a government report.

Survivors said they watched helplessly as the guard who had the keys fled without unlocking their cells.

"He threw the keys on the floor in panic," said Hector Daniel Martinez, who was being held as a homicide suspect.

Martinez said an inmate who was not locked in because he also worked as a nurse picked up the keys and, braving the scorching heat, went from one cell block to another, opening doors.

"He went into the flames and started breaking the locks," said Jose Enrique Guevara, who was five years into an 11-year sentence for auto theft. "He saved us, I tell you."

About 100 relatives of the presumed dead gathered in tight knots outside a barricade manned by police and soldiers at the Tegucigalpa morgue, their faces etched in pain as they waited for a public address system to call the name of the latest identified victim.

Family of the few victims identified by late Thursday were escorted to a cinderblock building where Red Cross workers in white suits unloaded black plastic body bags arriving from the prison farm about 55 miles away.

Most wore surgical masks to ward off the heavy scent of decaying bodies. Loads of cheap brown wooden caskets pulled up aboard pickup trucks, apparently donated.

Prosecutors' spokesman Melvin Duarte said 25 forensic examiners were working around the clock, starting with the few bodies that weren't too charred to have finger prints.

Prisoners who survived unscathed or suffered only minor injuries remained inside the prison. Those with more serious injuries were taken to hospitals and were trickling back Thursday. Some were being treated inside by the nurse credited with saving many lives.

Miguel Angel Lopez, a guard on duty inside the prison, said he called the fire brigade as soon as he saw the blaze, but it took firefighters 30 minutes to get inside.

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.