THE OBAMA administration’s charge that senior Iranian officials plotted to kill the Saudi ambassador in Washington was greeted with a considerable amount of skepticism in some quarters of Washington and the Middle East. Iran, argued some pundits, was unlikely to have undertaken such a brazen attack; it had little to gain by killing the ambassador; and, anyway, its clandestine operations were known to be far more skillful than the seemingly bumbling attempt to contract the assassination to a Mexican drug cartel.

Perhaps the doubters are right, and it is certainly prudent to reserve final judgment until all the facts of the case are known. But the FBI’s charging documents outline some substantial evidence, including the transfer of $100,000 to a bank account that the suspect arrested in the case, Mansour Arbabsiar, believed belonged to a Mexican cartel member. The FBI also recorded phone conversations between Mr. Arbabsiar and a man in Tehran he identified as an operative of the Quds Force of the Iranian Revolutionary Guard.

What’s more, the alleged plot is not a large leap from Tehran’s past acts. Its agents have previously assassinated dissidents in Europe. A former commander of the Quds Force — who is now Iran’s defense minister — is wanted for involvement in the 1994 bombing of a Jewish community center in Argentina. Iran left fingerprints all over the 1996 bombing of a U.S. military barracks in Saudi Arabia, and it hasn’t tried to hide the provenance of roadside bombs used to attack U.S. troops in Iraq and Afghanistan or rockets fired at the U.S. Embassy in Baghdad.

As for the Saudi ambassador, Adel al-Jubeir is no diplomatic apparatchik: He is a senior foreign policy adviser to King Abdullah, a key link in what have recently been troubled relations between Saudi Arabia and the United States, and, thanks to Wikileaks, a known advocate of aggressive U.S. action against the Iranian nuclear program. It’s not hard to imagine Quds commanders regarding him as a prime target in what they see as a covert war against both Saudi Arabia and the United States.

The Obama administration can hardly be suspected of manufacturing this story to seek out a confrontation with Iran. In fact, its evident eagerness to curtail U.S. military operations abroad and cut the defense budget may have encouraged some in Tehran to believe that an attack on U.S. soil would not risk a serious response. That’s one reason why it’s important that U.S. countermeasures go well beyond the small extensions of sanctions announced Tuesday. The administration is rightly seeking to mobilize new multilateral action against Iran, including through the United Nations. But there are steps that the United States and close allies can take, such as directly sanctioning Iran’s central bank — a measure that already has considerable support in Congress.

The alleged plot against the ambassador may reflect a splintering of the Iranian regime that allows radical factions to act more autonomously. It could be a reaction to the stress caused by biting economic sanctions and apparent covert operations by the West or Israel that have killed Iranian nuclear scientists and disabled centrifuges. Whatever the cause, the scheme’s discovery should serve as a warning of the escalating threat posed by Iran — and the need to act more forcefully against it.