TEGUCIGALPA, Honduras (AP) — 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 killing 355 people was absurd.

Details of the investigation remained thin early Friday, 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.

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.

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 (90 kilometers) 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.

The U.S. State Department has criticised the Honduran government for harsh prison conditions, citing severe overcrowding, malnutrition, and lack of adequate sanitation.