U.S. Ambassador to Mexico Ken Salazar met with President Andrés Manuel López Obrador in Mexico City on Monday as the condition and whereabouts of four Americans who were kidnapped at gunpoint remain unknown.
The unidentified U.S. citizens were ambushed Friday by gunmen who fired on the group shortly after they crossed from Brownsville, Texas, into Matamoros, Tamaulipas, according to the FBI. The Americans were driving a white minivan with North Carolina license plates.
The kidnappers moved all four into another vehicle before fleeing the scene.
Salazar noted that an innocent Mexican citizen was killed during the attack.
"We have no greater priority than the safety of our citizens – this is the US government's most fundamental role," the ambassador said in a statement.
Salazar said various law enforcement agencies are working with Mexican authorities "at all levels of government to achieve the safe return of our compatriots."
The FBI is offering a $50,000 reward for the return of the Americans and the arrests of those involved.
Mexico's president offers a theory on the kidnapping
López Obrador on Monday told reporters during his daily press briefing that the kidnapped group had crossed the border to buy medicines in Mexico.
He did not say how he or other officials came the conclusion, but he told reporters that he has been in contact with the governor of Tamaulipas.
"There was a confrontation between groups, and they were detained," López Obrador said. "The whole government is working on it."
He added: "I think it will get resolved, that's what I hope."
An unnamed U.S. official reportedly told CNN that the U.S. citizens had "traveled to the border city of Matamoros for medical procedures," citing receipts found in the abandoned vehicle.
FBI officials told NPR they would not comment on López Obrador's remarks nor on reports of the receipts.
White House press secretary Karine Jean-Pierre said the U.S. will "push [the Mexican government] to bring those responsible to justice."
"Clearly, we want to be really careful here. There are privacy concerns, and so I don't want to share too much information on how we're moving forward," Jean-Pierre told reporters.
Neither the White House nor the FBI have disclosed further information about the circumstances of the attack beyond the FBI's and the ambassador's earlier statements.
Drug cartels unofficially rule over Matamoros
For years, Matamoros has been a stronghold for various feuding criminal organizations, particularly the Gulf Cartel, which has used the city as a key pipeline for moving cocaine, meth and fentanyl across the border into Texas — and from there across the U.S.
Since 2010, when drug wars first started to break out between the factions, the city has been under unofficial control of the Gulf Cartel.
The state of Tamaulipas is considered one of the most violent places in Mexico. As recently as October the U.S. State Department issued a "Level 4: Do Not Travel" advisory, citing crime and kidnapping.
