Mexican President Claudia Sheinbaum has described the killing of six migrants by Mexican troops as “deplorable” and called for an investigation into the shooting.

“It’s a regrettable event, and it must be investigated and punished,” Sheinbaum said at a news conference on Thursday. “A situation like this cannot be repeated.”

She added that the attorney general’s office was investigating the incident but did not say whether any action had already been taken against the soldiers.

Her remarks come in response to an incident on the evening of October 1, when Mexican soldiers opened fire on a truck they described as travelling “at high speed” near the town of Huixtla, close to the border with Guatemala.

The Ministry of Defence later said the soldiers heard “explosions” coming from the truck, prompting their response.

Thirty-three migrants were on board the truck at the time of the shooting, according to the ministry. The six people who were killed were citizens of Egypt, Peru and El Salvador.

It was the worst killing of migrants and asylum seekers by authorities in Mexico since police in the northern state of Tamaulipas killed 17 migrants in 2021, and the resulting outcry from this attack has presented an early challenge for Sheinbaum, who was just sworn in this week.

Sheinbaum did not say how many migrants from each country had been killed, and Mexico’s Ministry of Foreign Affairs was not immediately able to provide details. Peru’s Foreign Ministry, however, confirmed one Peruvian was killed and demanded “an urgent investigation” into the killings.

The area between Mexico and Guatemala is a common route for smuggling migrants, some of whom are packed into crowded freight trucks to travel northward to the US border.

It has also been the scene of drug cartel turf battles, and the Defence Ministry said the trucks “were similar to those used by criminal groups in the region”.

Irineo Mujica, a migrant rights activist who has worked in the area, said he doubted the migrants or their smugglers opened fire first, prompting the Mexican soldiers’ response.

“It is really impossible that these people would have been shooting at the army,” Mujica said. “Most of the time, they get through by paying bribes.”

The incident, however, is not the first time Mexican law enforcement has claimed to have heard explosions or gunfire in the lead-up to the use of deadly force.

In 2021, the National Guard opened fire on a pick-up truck carrying migrants and asylum seekers, killing one and wounding four. Officials initially said some of those in the truck were armed and had fired shots, but an independent investigation later found that was not true.

Also in 2021, state police in Tamaulipas killed 17 migrants and two Mexican citizens. Those officers also claimed to have come under fire from the migrants’ vehicles.

The Roman Catholic Mexican Council of Bishops called the latest killings “a disproportionate use of lethal force”.

“This tragedy is not an isolated incident,” the council said in a statement. “Rather, it is the consequence of militarisation of immigration policy and the greater presence of armed forces on the country’s southern border.”

Former President Andres Manuel Lopez Obrador, whose term ended on September 30, expanded the authority of the military during his six years in office, folding the National Guard under its command and engaging troops to participate in civilian infrastructure projects.

Sheinbaum, his successor and fellow Morena Party member, is expected to carry forward his policies.

She did not mention the killing during a visit on Thursday to a Mexico City military base, where army and navy commanders pledged their loyalty to her in front of combat vehicles and hundreds of troops.

“In our country, there is not a state of siege. There are no violations of human rights,” Sheinbaum said during the visit, focusing instead on wage increases for the troops.