US and Mexican authorities have launched probes into "a case of mistaken identity" after four American citizens were assaulted and kidnapped in north-eastern Mexico late last week.
Mexican President Lopez Obrador said on Monday that his "whole government is working” on spotting the four, who have yet to be identified.
He was echoed by US Ambassador to Mexico Ken Salazar, who stressed that "officials from various US law enforcement agencies are working with Mexican authorities at all levels of government to achieve the safe return of our compatriots."
The statement came after a US media outlet cited an unnamed source as saying that the missing Americans are believed to have been targeted by mistake and were not the intended victims.
According to investigators, a Mexican cartel apparently mistook the four for Haitian drug smugglers after the Americans arrived in the border city of Matamoros for purported medical procedures last Friday.
Obrador for his part told reporters that "the information we have is that they crossed the border to buy medicines in Mexico, there was a confrontation between groups and they were detained." The FBI in turn suggested that the Americans were driving in a van with North Carolina license plates.
White House press secretary Karine Jean-Pierre has meanwhile said that President Joe Biden had been informed of the situation, and that that the White House was "closely following" the developments.
"These sorts of attacks are unacceptable. Our thoughts are with the families of these individuals and we stand ready to provide all appropriate consular assistance," she pointed out.
The US State Department earlier issued a "Level 4: Do Not Travel" advisory for those American citizens who plan to visit the Mexican state of Tamaulipas where Matamoros is located, citing crime and kidnapping. According to US government estimates, hundreds of thousands of Americans cross the border into Mexico annually to receive healthcare services, including prescription drugs. Most of them cite cheaper costs as the most common reason to get treatment in Mexico.