MEXICO CITY — Mexican police participated in a massacre last month that left 19 people dead, including at least 13 who appear to have been Guatemalan migrants on their way to the United States, a state prosecutor said late Tuesday.

The victims were killed Jan. 22 in Camargo, close to the Texas border. The bodies were found in a pickup truck, so badly burned that they could not be identified. Back in Guatemala, their families received a call from a smuggler: The migrants had been killed in a shootout, he said.

Irving Barrios Mojica, the attorney general of the northern state of Tamaulipas, said an investigation had proved that “at least 12 elements of the state police participated in the events of Jan. 22.”

The alarming finding comes as President Joe Biden attempts to reshape U.S. immigration policy so that it is “more fair, orderly and humane,” he said earlier Tuesday. The massacre in Camargo underscores not only the risks that migrants confront to enter the United States, but the destabilizing role that Mexico’s security forces sometimes play along the border.

At the Trump administration’s behest, Mexico mobilized its army and police to deter migration, despite objections from human rights groups. Allegations of police participation in the massacre will force Biden to reckon with the risks of continuing that policy.

The U.S.-Mexico border is closed to migrants, including asylum seekers, under a pandemic-era policy that Biden has so far upheld. That means the only way for most migrants to enter the United States is to pay a smuggler, as the Guatemalans did, putting their fate in the hands of a criminal group.

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The 12 officers have been detained on homicide charges. It remains unclear why the police would kill the migrants, or if the Guatemalans were mistakenly targeted. Camargo is near a stretch of northern Mexico that is contested by two major criminal organizations, the Zetas and Gulf cartels. It’s a major smuggling point for drugs and people.

Only two of the Guatemalan victims have been identified. Guatemalan officials say at least 11 others were also Guatemalan. Photos of the scene showed a pile of charred, skeletal human remains in the back of a pickup.

The massacre has already driven a wedge between Mexico and Guatemala.

“The Government of the Republic of Guatemala expresses its absolute rejection of the atrocities committed in the Tamaulipas massacre and expresses its deepest condolences to the families of the victims,” the Guatemalan government said in a statement Tuesday night.

On the day of the attack, Mojica said, vehicles passed through the area carrying migrants to the United States. One of those vehicles, he said, “carried armed individuals who provided protection for the group.”

The Guatemalan migrants were mostly young men and women trying to escape the grinding poverty of Comitancillo, a rural town in one of the poorest regions in Latin America. Relatives said at least one of the victims was a man who had been deported from the United States and was trying to return to his wife and child in Mississippi.

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In a local news report from Guatemala, Olga Perez, the mother of one of the victims, wept as she described what had happened.

“When she left she was normal, and when she returns she will be completely charred,” she said.

Mario Gálvez, a Guatemalan lawmaker whose district includes Comitancillo, wrote on Facebook that 13 of the victims were from his area. He issued a condolence letter with their names.

“I have already contacted the authorities of the Guatemalan foreign ministry to give all the necessary attention to families to support the repatriation of the bodies of these countrymen, victims of insecurity, in search of their dreams,” he wrote.

Edgar López y López was attempting to return to Jackson, Miss., where he had lived for 26 years, his family members told local reporters. They are now trying to raise money to pay for the repatriation of his body.

Another of the suspected victims was Rivaldo Jiménez Ramírez. His uncle told local reporters through tears: “People recognized him for the good kid that he was — obedient and hardworking.”

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In 2010, 72 migrants were massacred in San Fernando, another part of Tamaulipas state. Members of the Zetas cartel were linked to the attack. Some in Mexico have drawn a parallel between the two incidents, near the U.S. border and in one of the most dangerous stretches of the Western Hemisphere. But Mexican officials played down the similarities.

“This is not a San Fernando because we are advancing in the investigation,” Olga Sánchez Cordero, the national secretary of government, said last week. “There won’t be impunity.”

But now that the police have been accused of being involved in the massacre, Mexico will once again face profound questions about the security forces’ role in violence and human rights abuses.

Mexican authorities said evidence at the crime scene appeared to have been manipulated. One of the trucks, for example, was riddled with bullet holes, but investigators didn’t find any shell casings.

In 2019, the Trump administration pressured Mexico into using its military and police to deter migration. That pressure drew critics.

“Given the deplorable human rights record of Mexican security forces in recent years — and especially the military — it’s predictable that the deployment will result in serious abuses,” wrote Daniel Wilkinson of Human Rights Watch.