Two sisters from Texas and a friend are missing in Mexico after they crossed the border last month to sell clothes at a flea market, U.S. authorities said Friday.

Maritza Trinidad Perez Rios, 47; Marina Perez Rios, 48; and their friend, Dora Alicia Cervantes Saenz, 53, were traveling in a green mid-1990s Chevy Silverado to a flea market in the city of Montemorelos, in Nuevo Leon state. It’s about a three-hour drive from where the sisters are from in Peñitas, a small border city in Texas near McAllen.

Very few details have been released. The FBI said it is “unable to provide comment on this ongoing investigation” and “relentlessly pursues all options when it comes to protecting the American people, and this doesn’t change when they are endangered across the border.”