In front of portraits of the late Iranian revolutionary Ayatollah Khomeini, left, and supreme leader Ayatollah Ali Khamenei, President Mahmoud Ahmadinejad sings the Iranian national anthem in June. (Vahid Salemi/AP)

The straight-out-of-pulp-fiction plot by alleged Iranian operatives to assassinate a Saudi diplomat in Washington was so badly bungled that investigators initially were skeptical that Iran’s government was behind it, U.S. officials said Wednesday.

Officials laying out the details of the case owned up to their early doubts about an Iranian role as they sought to counter skepticism and confusion about the unusual scheme — one that happens to carry far-reaching international consequences.

Less than 24 hours after disclosing the disruption of the alleged plot, the Obama administration spent much of Wednesday outlining the evidence, not only to journalists but also to international allies and members of Congress. In briefings and phone calls, U.S. officials sought to explain how Iran’s vaunted Quds Force allegedly ended up enlisting a used-car salesman and a Mexican drug gang in a plan to kill Saudi Arabia’s U.S. ambassador and blow up embassies in Washington and Buenos Aires.

Western diplomats who were privately briefed by U.S. officials at U.N. headquarters in New York said the Americans expressed concern that the plot’s cartoonish quality would invite suspicions and conspiracy theories. “Everyone was surprised by the amateurishness of the plotters,” said one U.N. Security Council diplomat, speaking on the condition of anonymity under diplomatic protocol.

Although Justice Department officials say they convincingly linked the assassination plan to “elements of the Iranian government” — specifically the Quds Force — U.S. officials acknowledged that the case bore few of the hallmarks of a unit that has trained and equipped militants and assassins around the world.

“What we’re seeing would be inconsistent with the high standards we’ve seen in the past,” said a senior U.S. official, one of four who briefed reporters on the case. The officials agreed to speak on the condition that their names and professional affiliations not be revealed.

Many of those involved in the case identified a long list of improbables that argued against official Iranian ties to the alleged plot. It was out of character for Iran to undertake such a risky mission, and it strained credulity to imagine how professional operatives would stoop to hiring unknown drug-cartel members for a high-level political assassination.

“We had to be convinced,” the official said.

After months of undercover work, investigators began to see compelling evidence — including money transfers from Iran — that linked the plot to the Quds Force. While acknowledging that they did not have conclusive proof, the U.S. officials said they believed that Quds Force chief Qassem Suleimani and Ayatollah Ali Khamenei, Iran’s supreme leader, were at a minimum aware of the scheme’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 about the fine details of the alleged 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. They said the oddly brazen nature of the plot may also reflect the naivete of the hard-line clerics who have 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. The official said it’s not unusual for Iran to form alliances of convenience with groups that do not share its ideology or worldview.

The administration began the work of marshaling international support for harsher measures against Iran, although it was not clear what those would be.

Secretary of State Hillary Rodham Clinton blasted Iran for what she called a “dangerous escalation” of the country’s decades-long tradition of supporting assassinations and terrorism overseas, and she called on allies to help increase Iran’s diplomatic and economic isolation.

“Iran must be held accountable for its actions,” Clinton said.

Clinton and U.N. Ambassador Susan E. Rice expanded on the evidence in private conversations with their foreign counterparts, Western diplomats said. A French diplomat, who also spoke on the condition of anonymity, said the information presented by Rice and the American team was “credible and convincing.”

The administration is under pressure from Congress to take firm action against Iran, beyond new economic sanctions announced Tuesday. Suggestions ranged from sanctions targeting Iran’s central bank to military exercises off its coast.

“We’re continuing to look at what more we might be able to do,” said State Department spokeswoman Victoria Nuland.

Justice Department officials disrupted the alleged 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 the hit against Adel al-Jubeir, the Saudi ambassador to the United States.

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

During a four-month undercover probe, Arbabsiar’s Quds Force contact wired nearly $100,000 to the agent, according to court documents. Arbabsiar was also induced after his arrest to phone his Iranian sponsor while American agents listened in, U.S. officials said.

In Tehran, the U.S. accusations have been dismissed by government spokesmen as fabrications intended to isolate Iran and distract public attention from U.S. economic worries. But Iranian analysts agreed that even if U.S. charges of official Iranian involvement were true, President Mahmoud Ahmadinejad and his government probably had nothing to do with the scheme.

The security organizations that the United States says were behind the alleged plot — the Quds Force and its parent, the Revolutionary Guard Corps — are well beyond Ahmadinejad’s influence. Leaders associated with those entities have played key roles in attacking Ahmadinejad during his rift with powerful Shiite Muslim clerics and commanders who helped bring him to power.

Amid new levels of infighting within Iran’s opaque leadership, Ahmadinejad at present wields little or no influence over the country’s two main intelligence and security organizations: the Ministry of Intelligence and the Revolutionary Guard Corps. They are under the control of Khamenei, the supreme leader.

Even against the backdrop of this power struggle, Iranian dissidents and analysts were hard-pressed to come up with reasons for any of Iran’s leaders to undertake such a risky plot. Even if carried out successfully, it probably would have been quickly blamed on Iran, the analysts said.

Some suggested various possible culprits in the alleged plot, ranging from the CIA to Revolutionary Guard elements to a rogue faction within Iran’s power structure.

“There are those within the Guards with some degree of independence,” said Sadegh Zibakalam, a political scientist critical of the government. “But I cannot point any fingers in this bizarre plot that only hurts Iran.”

Erdbrink reported from Tehran. Staff writers Scott Wilson, Jerry Markon and Rosalind S. Helderman in Washington and Colum Lynch at the United Nations contributed to this report.