A forensic team that included U.S. experts examined the charred interior of a Paraguay (search) supermarket Wednesday to determine the cause of a weekend blaze that killed more than 400 people, many of whom were trapped inside by locked doors.

As the specialists took burn samples from the building, Interior Minister Orlando Fiorott said the investigation "clearly points" to an accidental gas leak that ignited. He said that it didn't look as if Sunday's blaze had been intentionally set, but cautioned that the findings were preliminary.

The death toll was revised to 426 on Wednesday, down from 464 a day earlier; 520 people remained hospitalized with burns and other injuries. The attorney general's office said 153 were reported missing.

Officials charged a co-owner of the supermarket and four others with manslaughter Tuesday after a security guard said he was ordered to lock the doors to prevent people from stealing.

Officials have said they were checking reports that an exploding gas canister could have started the flames, which forced a floor to collapse, crushing cars and burning many bodies beyond recognition.

President Nicanor Duarte has called for a swift and thorough investigation into the tragedy at the Ycua Bolanos supermarket, food court and parking garage in suburban Asuncion (search), the Paraguayan capital.

The charges came after chief investigator Edgar Sanchez said a security guard testified he was told via radio to lock the doors when the fire began. Sanchez said the guard didn't know who gave the order.

Meanwhile, Paraguayans continued to mourn their dead.

Outside a nightclub near the supermarket that became a makeshift morgue, school-age children held a vigil and lit candles to remember the victims.

"One of my best friends was killed in the fire and I miss her terribly," said 12-year-old Ana Benitez. "She sat next to me in class and it's going to be painful when school resumes and I see that empty chair next to me."

Paraguayan officials said they've begun reviewing leading shopping centers in the capital and their emergency preparations.

Angel Villalba, the president of the Paraguayan Association of Supermarkets (search), told a radio station that initial findings have been alarming.

"Almost none of them have emergency exits," he said.