U.S. officials are baffled but pleased that U.S. ships in the Persian Gulf have stopped being harassed by armed Iranian fast boats, military officials from the US Central Command said.

The boats for at least two years would dart toward the U.S. vessels as they passed through the Persian Gulf, risking miscalculation, but haven’t done so for five months, U.S. military officials said.

The officials said they hoped the respite would continue. “I hope it’s because we have messaged our readiness…and that it isn’t tolerable or how professional militaries operate,” Army Gen. Joseph Votel, who heads U.S. Central Command, told reporters traveling with him in the Middle East this week. Iranian officials didn’t respond to a request for comment.

The fast boats are typically armed with .50 caliber machine guns and rocket launchers and have come within shooting distance of American naval vessels, encounters that grew routine even though each one presents potential dangers to American vessels transiting through international waters.