The United States is home to several species of agave, the most common being Agave Americana.
There has not been a documented history of much production of agave spirits, but recently there has been a surge of interest in creating an agave spirits category in the United States. Several projects have imported agave juice from Mexico to distill here, but more recently there have been several groups planting agave with the deliberate plan to distill. Any agave spirit produced outside the Denominacion de Origen (DO) for mezcal cannot be labeled as Mezcal. This is true for tequila as well. Rather they will be labeled as an Agave Distillate or Destilado de Agave.
Additionally, there are companies that producing Sotol, and while not made from an agave, the production process is very similar to mezcal they are often lumped in together. While Sotol does have a DO, it is not yet recognized internationally and therefore you are able to find bottles labeled Sotol that have been produced in the United States.