Soon after the March attack, Rex Tillerson, then the U.S. secretary of state, ordered State Department officials to outline the case for designating Russia as a state sponsor of terrorism under U.S. law. Experts in the department’s Bureau of Counterterrorism began to assemble what they thought was a strong case.

But almost as quickly as the review began — within about two days — the secretary of state’s office sent new instructions to drop the initiative, according to State Department officials familiar with the episode.

“There are a lot of issues that we have to work on together with Russia,” a U.S. official said. “Designating them would interfere with our ability to do that.”

The State Department’s reluctance to impose the terror designation was not a product of Trump administration sympathy for Russian President Vladimir Putin, U.S. officials say. Rather, it reflected an ambivalent and at times contradictory policy toward Russia on terrorism issues that stretches back more than a dozen years, American intelligence officials and foreign-policy experts said.