A search is under way for four US citizens kidnapped in north-eastern Mexico last week after apparently getting caught up in a shootout which left one Mexican dead.
They were driving through Matamoros in a white minivan when unidentified gunmen opened fire.
Video shows them being put in a pickup truck by heavily armed men.
Mexican President Andrés Manuel López Obrador says the Americans crossed the border to buy medication.
But his statement was not confirmed by US officials, who have not named the four people missing.
President López Obrador said his “entire government” was working to obtain the release of the four US citizens.
He held talks with US Ambassador Ken Salazar at the National Palace in Mexico City on Monday.
“There was a confrontation between groups and they were kidnapped,” the president said.
The incident also resulted in the death of an innocent Mexican citizen, Mr Salazar said.
A video posted on social media shows gunmen loading four people on to the bed of a pickup in broad daylight. One is manhandled on to the vehicle while others appear to be unconscious and are dragged to the truck.
