Iran appears to have scaled back testing of its nuclear-capable ballistic missiles last year due to a threat from the Trump administration, according to a new study by the Foundation for the Defense of Democracies.

Tehran conducted 23 total missile tests since signing the U.S.-broker nuclear deal in 2015. The most tested variant had been the medium-range missiles that are considered most threatening because they can carry a nuclear warhead and reach U.S. bases in the region, the study found.