The U.S. Centers for Disease Control and Prevention issued a report Thursday about three cases of infections apparently linked to stem-cell treatments American patients received in Mexico.

The CDC issued the report Thursday on infections of Non-Tuberculous Mycobacteria (NTM), which it described as “difficult-to-treat” and “intrinsically drug-resistant” and “rapidly growing.”

All three patients detected at two Colorado hospitals remain in treatment. The infections were apparently acquired during stem-cell injections carried out at two different clinics in the Mexican border state of Baja California located 167 miles (269 km) apart.

In the past, Americans and Mexicans have been infected apparently because some doctors in Mexico were taking multiple doses of anesthetics from a single vial due to shortages or other problems, though it was not clear if that was a possible cause in the most recent outbreak reported Thursday.