The plot by alleged Iranian operatives to kill a Saudi diplomat in Washington was so crudely constructed that U.S. investigators initially had trouble believing that Iran was truly behind it, U.S. officials said Wednesday.

Mansour Arbabsiar (CBS News)

A wire transfer two months into the uncover probe--$100,000 in cash, moved in tell-tale fashion from Iranian bank accounts to an undercover agent in Mexico--finally persuaded American investigators that the assassination plan had high-level backing. And still, questions remained about who in Iran knew of the plot and at what level it was approved.

“What we’re seeing would be inconsistent with the high standards we’ve seen in the past,” said one U.S. official, one of four who briefed journalists about the four-month investigation. The officials agreed to speak on the condition that neither their names nor affiliations be disclosed.

Justice Department officials disrupted the plot in September with the arrest of an Iranian-American, Mansour Arbabsiar, 56, who is accused of working with Quds Force members in Iran to carry out a hit against Adel al-Jubeir, the Saudi ambassador to the United States.

According to court documents, Arbabsiar was tasked by an Iranian cousin with recruiting Mexican hit-men for a $1.5 million plan to kill Jubeir as he dined in a Washington restaurant. The alleged plan was foiled when Arbabsiar made contact with a man he believed was a drug-cartel member. Instead, it was an undercover informant for the U.S. Drug Enforcement Administration.

The U.S. officials acknowledged that initial details of the alleged plot engendered great skepticism among law-enforcement and intelligence analysts who worked on the case. The Iranians involved exercised uncharacteristically sloppy tradecraft in trying to recruit unknown gunmen—from a drug cartel with no known ties to Iran—to carry out such a politically explosive act as the assassination of a powerful Saudi envoy in the heart the U.S. capital.

Over time, however, intelligence agencies gathered what they considered corroborating evidence connecting the plot firmly to Quds Force officers, including Gholam Shakuri, a member of the elite unit with whom Arbabsiar met in Iran. The officials declined to elaborate on the nature of the evidence but acknowledged that the money transfer provided important clues.

While acknowledging they did not have conclusive proof, the U.S. officials said they were convinced that Quds Force chief Qassem Suleimani and Iranian Supreme Leader Ayatollah Ali Khameinei were at least aware of the plot’s general outlines.

“We do not think it was a rogue operation, in any way,” a second official said. But he added: “We don’t have specific knowledge that Suleimani knew about specific” details of the plot.

The officials said American investigators theorized that the operatives’ sloppiness reflected Iran’s inexperience in working in North America, where even the globally networked Quds Force lacks connections and contacts. But they said the oddly brazen nature of the plot may also may have reflected the naivete of the clique of hard-line clerics that has come to dominate Iran’s leadership in recent years.

“These leaders have no Western experience, and they have a great misunderstanding of the United States,” the second official said. “They don’t understand where the red lines are.”