When U.S. officials first learned of the alleged plot, “there were significant doubts there was any ‘there’ there,” the official said. But “multiple sources of independently verified information” corroborated the account, the official said. “It coalesced into a picture of something unusual but serious,” the official said.

The official said a key indicator that the plot was real was the wire transfer of large sums of cash.

The United States is offering “specialized briefings” to officials in other countries who have sought more information about the alleged plot, said State Department spokesman Victoria Nuland. She cited Russian Foreign Minister Sergey Lavrov as having wanted more information.