Two US citizens kidnapped by suspected Mexican drug traffickers were found dead Tuesday while two others survived, in what officials said appeared to be a tragic case of mistaken identity.

Washington vowed to do everything in its power to ensure justice for the victims, who crossed the border into the crime-plagued city of Matamoros in Tamaulipas state on Friday for medical reasons.

Initial indications were that the abduction was the result of a "mix-up" rather than a targeted attack, Tamaulipas Attorney General Irving Barrios told reporters.

The two survivors were returned to the United States via a land border crossing between Matamoros and Brownsville, Texas, officials said.

US media named them as Latavia Washington McGee and Eric James Williams.

One suffered a gunshot wound to the leg and the other was unharmed, according to Tamaulipas Governor Americo Villarreal.

The victims visited Mexico because one of them was planning to have cosmetic surgery, he said.

It was not immediately clear whether the two deaths occurred before or during the rescue operation, but Villarreal said preliminary investigations suggested all four were alive until at least Monday.

"During the three days after the criminal act, the four people deprived of their liberty were transferred to various places, including a clinic, in order to create confusion and avoid rescue efforts," he said.

The bodies of the two US citizens who lost their lives were expected to be repatriated within hours once forensic studies were completed in Mexico, Villarreal added.

- 'Relentless' US pursuit -

The victims had traveled to Matamoros on Friday in a white minivan with North Carolina license plates, the US Federal Bureau of Investigation (FBI) said in an earlier statement.

It offered a $50,000 reward for help leading to the return of the unidentified victims and the arrest of the perpetrators.

"Shortly after crossing into Mexico, unidentified gunmen fired upon the passengers in the (minivan). All four Americans were placed in a vehicle and taken from the scene by armed men," the FBI said.

A 33-year-old Mexican woman died near the scene, possibly from a stray bullet, Mexican officials said.

The White House denounced the kidnappings as "unacceptable" and offered condolences to families of the victims.

"We're going to work closely with the Mexican government to ensure that justice is done in this case," White House national security spokesman John Kirby said.

US Attorney General Merrick Garland said the Justice Department would be "relentless" in pursuing justice on the victims' behalf.

"We will do everything in our power to identify, find, and hold accountable the individuals responsible for this attack on American citizens," he added.

Mexican authorities said they had arrested a suspect who was guarding the abductees at a house in Matamoros.

- Cartel hot spot -

"We're very sorry that this is happening in our country," Mexican President Andres Manuel Lopez Obrador told reporters, offering condolences to the families of the victims.

"Those responsible are going to be found. They are going to be punished," he added.

Tamaulipas is one of the Mexican states most affected by drug trafficking and other organized crime.

The US ambassador to Mexico, Ken Salazar, said that the abduction was "a tragic reminder" that both countries must "strengthen the fight against criminal organizations" along their shared border.

The Latin American country is plagued by cartel-related bloodshed that has seen more than 340,000 people murdered since the government deployed the military in the war on drugs in 2006.

The US State Department advises against travel to Tamaulipas due to dangers including "gun battles, murder, armed robbery, carjacking, kidnapping, forced disappearances, extortion, and sexual assault."

Despite the risks, Matamoros, located on the banks of the Rio Grande river separating the two countries, is a major stopping point for irregular migrants heading for the United States.