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.