MEXICO CITY — Zalayna Grant last spoke to her brother on Thursday, when he told her he was driving to Mexico with a group of friends — one of them was planning to get a tummy tuck there.
Her brother, 28-year-old Zindell Brown, was laughing, Ms. Grant said, and she could hear his three friends in the car, having a good time.
“They weren’t nervous,” Ms. Grant, 34, said in an interview.
But by Tuesday, the Mexican authorities confirmed that her brother was dead, one of the two Americans in the group who were killed after gunmen attacked their car hours after they had crossed into the dangerous border town of Matamoros, in Tamaulipas state last week.
The two others in the group survived — one suffered a gunshot wound in his leg. They were found along with the bodies of the dead in a house outside the city, where they were being held, the authorities said.
The Americans were the latest victims of relentless violence in Mexico that the government has been unable to contain despite promises from the country’s president that his security strategy is working.
It comes at a critical moment for the security relationship between Mexico and the United States, as Republican members of Congress have proposed a bill that would allow the U.S. military to combat organized crime south of the border and as several states push to label drug cartels terrorist groups.
“It feeds a narrative that Mexico is a lawless place, that Mexico has no capabilities of its own to deal with this and that the U.S. needs to do something,” said Alejandro Hope, a security analyst in Mexico City. “The Biden administration may come under increasing pressure to do something.”
Ms. Grant said that along with her brother, Mr. Brown, the three other kidnapping victims were Latavia Washington McGee, Shaeed Woodard and Eric James Williams.
The Mexican authorities said Ms. McGee and Mr. Williams had survived, and were transferred to U.S. officials on Tuesday.
Investigators are considering many possible explanations for the attack, but have focused on the theory “that there was a confusion, that it wasn’t a targeted attack,” Irving Barrios, the state prosecutor in Tamaulipas, said at a news conference on Tuesday.
“That’s the line that we have right now as the most viable,” Mr. Barrios said.
Two Mexican officials familiar with the investigation who were not authorized to speak publicly said that among the various motives being considered by law enforcement is the possibility that the Americans were mistaken for smugglers of Haitian migrants.
Illegal migrant crossings at the U.S. southern border have soared in recent months, spurring tension among criminal groups that control human trafficking through northern Mexico, experts said.
Karine Jean-Pierre, the White House press secretary, said on Tuesday that the Biden administration was in touch with the families of those kidnapped. “We will continue to work closely with the Mexican government to make sure justice is done in this case,” she said.
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She said the administration has worked to disrupt cartels through sanctions imposed on criminal groups. “Our immediate concerns are for the safe return of our citizens,” she added.
Ms. Jean-Pierre said the State Department travel advisory for the area where the Americans were kidnapped remains at Level 4, meaning “do not travel due to crime or kidnapping.”
“We urge Americans to read these alerts before traveling,” Ms. Jean-Pierre said.
She declined to answer question about whether Mr. Biden would consider using military action against the cartel.
The four Americans drove into the border town of Matamoros, Mexico, from Brownsville, Texas, in a white minivan on Friday when gunmen began firing on their vehicle, the F.B.I. said. The gunmen then put the Americans in another vehicle and drove them away.
During the initial confrontation between the victims and the kidnappers, “an innocent Mexican citizen” was killed, according to Ken Salazar, the U.S. ambassador to Mexico.
Américo Villarreal, the Tamaulipas governor, said the authorities found “medical laboratory tests” in the Americans’ car that seemed to indicate “that one of the Americans had come to undergo cosmetic surgery.”
On Sunday, Ms. Grant saw a video circulating on social media of four people being pulled onto a pickup truck. She recognized Ms. McGee’s blond braids, she said, as well as her brother, who she said was wearing tan pants and a black shirt. “I thought, that’s my brother right there,” she said.
One of the other kidnapped Americans, Mr. Woodard, had recently worked at an insulation company and then cleaning rooms at an oceanside hotel in Myrtle Beach, S.C., according to former colleagues. “He was always willing to work and always very friendly,” said Regina Downs, a manager at the Monterey Bay Suites.
Michael Graham, 45, said he was friends with three of the four abducted Americans, and that they were known around Lake City, a town of 6,000 in central South Carolina, as a foursome, so he was not surprised they had gone to Mexico together.
“They went everywhere together,” Mr. Graham said. “You saw one, you saw all four.” At least some of them had recently moved to Myrtle Beach, he said.
While Americans sometimes get entangled in violence in northern Mexico, a shared border nearly 2,000 miles long with large swaths dominated by drug cartels and criminal organizations, it is unusual for U.S. nationals to be kidnapped in Mexico.
The widely shared video that appears to be of the kidnapping showed three men dragging people on the ground and then lifting and dropping them in the bed of a white pickup truck. At least one of the men wore an armored vest, and they were dragging the people in clear view of nearby traffic.
The U.S.-Mexico border is one of the busiest in the world, with young Mexicans crossing north to shop or attend private high schools, and American nationals going south to buy cheap medication or undergo medical procedures that are unaffordable at home, from dentist appointments to cosmetic surgery.
When Americans become victims of violence in Mexico, it is often because they are at the wrong place at the wrong time, traversing a frontier rife with criminal activity. The criminal groups that operate throughout the country often avoid targeting American nationals, fearful of the blowback it will cause with U.S. law enforcement.
In 2021, more than 200 Americans died in Mexico, State Department figures show, but most of those deaths were either accidents, suicides or drug-related. Only 75 were ruled homicides.
Mexicans must contend with the reality of unyielding violence every single day. More than 30,000 people are murdered every year in Mexico, according to government figures, and the vast majority of crimes go unsolved.
There have been some recent cases of particularly brutal violence against Americans in Mexico. The body of a U.S. public defender who had 40 skull fractures was found at a beach resort in northern Mexico in January.
That same month, Mexican authorities found four bodies, including that of an American architect who had been missing, near a bullet-riddled van in central Mexico. In October, a 25-year-old American woman was killed near the resort town of Cabo San Lucas.
And just a few days before the four Americans were kidnapped, an American citizen was killed by the Mexican military in the same state, Tamaulipas, as he was driving back from a nightclub with his friends on Feb. 26.
Elda Cantú, Oscar Lopez and Zolan Kanno-Youngs contributed reporting. Kitty Bennett contributed research.