The yellow transit bus rumbled between two slums on a muddy road lined by rusting warehouses and sugar-cane fields in San Pedro Sula, an...

The yellow transit bus rumbled between two slums on a muddy road lined by rusting warehouses and sugar-cane fields in San Pedro Sula, an industrial city about 100 miles from the Honduran capital of Tegucigalpa.

It was late evening last December, and among the Christmas shoppers on board were warehouse worker Emilio Lopez and his 10-year-old son.

Six men in a van raked the bus with automatic-weapons fire. As passengers screamed and ducked, a gunman climbed aboard and methodically fired away, authorities said.

When the shooting stopped, 28 people were fatally wounded. One was Lopez. He died apparently shielding his son, Emilio, who was found wounded and hiding under a seat, the boy’s mother, Maria Lopez, recalled.

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“These people have no souls,” she said of the gang known as Mara Salvatrucha, or MS-13.

Honduran President Ricardo Maduro blamed the group for the slaughter, saying it was a response to his administration’s “zero-tolerance” campaign, which has resulted in the arrest of more than 1,800 gang members since 2002.

The suspected mastermind of the bus attack is Lester Rivera-Paz, who is tied to an original MS-13 cell in Los Angeles, the Normandie Locos. He had been deported from the United States four times.

Known as El Culiche, or the Tapeworm, Rivera-Paz had a lengthy criminal record in California, including an armed robbery in 2000. The case was dropped when prosecutors could not find the victim, court records and interviews show.

Maduro has framed the struggle against MS-13 and other gangs as a fight for the life of his nation. Authorities said the gang plotted last year to assassinate Maduro and kill the president of Honduras’ Congress with a grenade.

Human-rights groups have accused the Honduran government of unjustified arrests and of tolerating death squads that have killed hundreds of gang members. Honduran Public Security Minister Oscar Alvarez said gang members may have been targeted. But he added that people are fed up with the violence.

Tough anti-gang measures have not always worked as planned. A month after his arrest, Rivera-Paz, the suspected bus-massacre mastermind, broke out of a Honduran prison.

Earlier this year, he was found hiding in the trunk of a Dodge Intrepid loaded with illegal immigrants as it raced north through Texas before dawn.

He pleaded guilty to illegally re-entering the country and is likely to serve two years in a U.S. prison.

Honduran officials agreed to let him serve out his prison term in the United States and said he will face charges in their country after he is deported — again.