MATAMOROS, Mexico — About 11:30 a.m. on Friday, Claudia said she was working in her office when she heard a staccato of sharp cracks — pop-pop-pop. She peeked outside to find heavily armed men shooting at a white minivan.
She saw a body being dragged along the street and ducked to avoid windows. Claudia, who grew up in the area, insisted that she be referred to only by her first name for fear of retribution.
At an elementary school nearby, the teachers, who are used to the sound of bullets outside, yelled at students to hit the floor, another witness said.
The chaos that unfolded at the busy, three-way intersection in downtown Matamoros, Mexico, might have been remembered as just a distressingly common act of violence in a violence-ridden border town — except this time, the victims were American.
After the nationality of the people who were attacked and abducted by gunmen that day became widely known, the Mexican president promised to put the force of his entire government behind the desperate effort to find them.
A task force made up of the police, the armed forces and the local authorities searched multiple locations and used security camera footage to track several vehicles. It also secured the assistance of American law enforcement officials.
The coordinated push led to an outcome that counts as extraordinary in Mexico: By Tuesday morning, just four days after the abduction, the Mexican authorities had recovered the victims, two dead and two alive, and had detained a suspect.
The speed of the rescue elicited anger among many Mexicans, who were shocked to see their leaders spring into action after years of doing little to locate the more than 100,000 people who remain missing in a country where the vast majority of crimes go unsolved.
“If only our government would apply the same force and due diligence to search for our disappeared in Mexico,” Delia Quiroa, who has been searching for her missing brother for nearly a decade, said in a video posted on Twitter.
The kidnapping and killings of Americans incited significant blowback in the United States, which has seen its security relationship with Mexico tested in recent years. Some Republicans in Congress seized on the violent episode to accuse the Biden administration of not doing enough to confront cartels in Mexico.
Border security throughout President Biden’s time in office has been one of the more glaring political vulnerabilities for his administration.
“This is a terrifying glimpse into how Americans can quickly fall victim to cartel violence and how emboldened the cartels are under the Biden administration,” said Representative Mark Green, Republican of Tennessee and chairman of the House Committee on Homeland Security. “The cartels continue to exploit the weak posture that the Biden administration has taken with its perilous border security policies.”
Senator Lindsey Graham, Republican of South Carolina, this week said he planned to introduce legislation to categorize Mexican cartels as terrorist organizations and allow the United States to use military force against them, a proposal that has been generally criticized by the Mexican president.
The four Americans who were attacked, Latavia Washington McGee, 33; Shaeed Woodard, 33; Zindell Brown, 28; and Eric James Williams, 38, a close-knit group of friends, traveled to Mexico from South Carolina last week.
They were accompanying Ms. McGee, who planned to get a tummy tuck at a clinic in Matamoros, in Tamaulipas State, family members said, making a journey popular among Americans who cross into Mexico for medical procedures.
Hours after they crossed into Mexico on Friday, they were targeted by gunmen in what the Mexican investigators believe was a case of mistaken identity.
The Mexican authorities are investigating the possibility that before the shooting began, armed men had told the Americans to stop their car but the driver instead sped up, according to two people familiar with the investigation who were not authorized to speak publicly.
Two of the victims, Ms. McGee and Mr. Williams, were found alive and returned to U.S. soil on Tuesday. The bodies of Mr. Woodard and Mr. Brown remain in Mexico.
Latonya Williams, Mr. Woodard’s fiancée, said in an interview that he did not usually travel far from home, but decided to go to Mexico “to support” Ms. McGee, his cousin, and to celebrate his birthday, on March 9.
“He was like, ‘My birthday was coming; this would be a trip for my birthday,’” Ms. Williams said. Her fiancé, she said, never hesitated to offer others help. “He would give you the shirt on his back,” she said. “He would give you his last dollar if you needed it.”
The killings have ratcheted up pressure on the Biden administration over its approach to the ongoing violence south of the U.S. border. Part of the problem, former officials said, is that security cooperation with Mexico has faced challenges under President Andrés Manuel López Obrador of Mexico.
After American officials arrested a former Mexican secretary of defense, Salvador Cienfuegos, in 2020, Mexico threatened to kick Drug Enforcement Administration agents out of the country.
Gil Kerlikowske, a former commissioner of the U.S. Customs and Border Protection under the Obama administration, said he often hears from his peers in the D.E.A. that “there’s just no exchange of vital, timely information, and you have nobody to blame except President López Obrador.”
The Biden administration has been reticent to openly criticize Mr. López Obrador, fearing that provoking the Mexican leader could jeopardize his cooperation on stemming the flow of migrants to the U.S. border.
But many in Washington, concerned about violence and potential threats to democracy in Mexico, argue that the time has come for the U.S. government to begin publicly discussing its differences with Mr. López Obrador.
“The United States needs to be speaking a little more honestly about its relationship with Mexico,” said John Feeley, former deputy chief of mission in Mexico under the Obama administration. “Things are not going well.”
Mr. López Obrador has said that his administration continues to cooperate closely on security with the American government.
The White House on Wednesday said it did not endorse classifying the cartels as terrorists. Karine Jean-Pierre, the White House press secretary, said the classification would not grant the United States any authority that it did not already have and noted that the administration had already used sanctions designed to combat narcotics organizations.
Ms. Jean-Pierre also said on Wednesday that the administration was in contact with the families of the four Americans.
“We’ll do everything in our power to identify, find and hold accountable individuals personally responsible for the attack,” Ms. Jean-Pierre said.
Tamaulipas is one of the six states that the State Department warns Americans not to visit because of crime, though violence there has decreased in recent years. Homicides dropped from about 1,500 in 2012 to about 730 in 2021, government figures show.
Different factions of the powerful Gulf cartel dominate the state, and experts said that conflict between local criminal groups might have played a role in the attack.
“It sounds like you had a local faction that basically made a mistake, and they thought it was either probably another faction within the city or maybe a group that was coming in from outside the city,” said Robert J. Bunker, an expert on Mexican organized crime and director of research and analysis at a security consulting firm, C/O Futures, in California. “These poor American tourists kind of got caught in the middle.”
At the clinic where Ms. McGee was scheduled to have surgery on Friday, a nurse who would give only her first name, Veronica, because she feared for her safety, said that about half the patients who come in are American. The clinic reaches out to potential American clients with targeted ads on Instagram, she said, featuring before and after videos and photos of their patients.
Veronica said she remembered Ms. McGee from about two years ago, when she hired Dr. Roberto Chavez to perform another plastic surgery.
As many clients are wary of Matamoros’s reputation as a violent and lawless place, the clinic was intentionally located just four minutes’ walking distance from the crossing into Brownsville, Texas. Even so, Ms. McGee never made it to her 7 a.m. appointment.
Maria Abi-Habib reported from Matamoros, Mexico; Natalie Kitroeff and Oscar Lopez from Mexico City; and Zolan Kanno-Youngs from Washington. Jacey Fortin contributed reporting from New York, and Dylan Ortuno from Lake City, S.C.
