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.

But a year ago, then-national security adviser Michael Flynn stepped in front of the news cameras at the White House and declared that Iran was “on notice” following missile tests that the administration considered highly provocative.

Since then, Tehran only conducted one test of a medium-range missile, according to Behnam Ben Taleblu, the study author who is a senior Iran analyst at the foundation. The study relied on open-source information including Iran state media and western media reports to calculate the tests.

“It is highly likely that the administration’s threat intimidated Tehran, altering its flight-testing calculus,” he wrote.

An “hardline” Iranian outlet reported an Islamic Revolutionary Guard Corps official had complained that a missile test was delayed because there were fears of the potential U.S. response.

Overall, Iran appears to have cut its tests of nuclear-capable missiles in half, with four or five tests last year versus 10 or 11 tests in 2016, Taleblu found.

The Joint Comprehensive Plan of Action signed in 2015, known as the Iran nuclear deal, seeks to block Tehran from developing a nuclear bomb but does not address the missile testing.

The Trump administration has called the oversight a major flaw in the deal and President Trump is threatening to pull the United States out.

Trump is pressing European nations who also signed the deal to help pushed back on Iran’s missiles, its weapons exports and what military officials have long referred to as malign influence in the region.