The alleged Iranian plot to use Mexican cartel gunmen to assassinate the Saudi ambassador in Washington is one of the strangest, most serious terrorism cases to surface in years, a mix of seemingly credible evidence and unlikely scenarios that departs dramatically from Iran’s past record of global terrorist activity.

On Tuesday, a grim-faced U.S. attorney general and the FBI director accused Iranian intelligence officials in an alleged $1.5 million scheme to kill Ambassador Adel al-Jubeir of Saudi Arabia in a bombing at a restaurant in the capital.

The federal indictment has escalated an already fierce conflict between the United States and Iran, alleging a brazen decision by Iranian officials to shed blood on U.S. soil and an ominous convergence of threats from separate worlds: Iran’s far-flung terror apparatus and the Zetas, a drug cartel founded by former Mexican commandos.

The evidence seems strong in some ways. Investigators tracked wire payments amounting to nearly $100,000 allegedly from the Quds Force, the foreign operations unit of Iran's Islamic Revolutionary Guard Corps. They caught a suspected Iranian officer on tape giving orders to a Texas operative working with a supposed representative of the Zetas who, in reality, was a U.S. Drug Enforcement Administration informant.

The alarming charges reinforce concerns among Western government officials and experts about signs of growing activity by Iran and its proxy, the militant group Hezbollah, in Latin America.

“Hezbollah and Iran have been masters at identifying existing organized crime groups in front line areas and exploiting them,” Michael Braun, a DEA former operations chief who led investigations of the nexus between drugs and terror, said in an interview. “Hopefully, this is going to be a turning point for many in government regarding what the DEA has been saying about Iran and Hezbollah for the past five years.”

But the account in the indictment clashes with the past behavior of the Zetas and the Quds Force. Both the cartel and the espionage unit have calibrated their murderous operations to avoid direct confrontation with the United States. Although the masterminds of the five-month-long plot seem powerful and dangerous, the plot as described by prosecutors unfolded atypically.

“If it weren’t for things like large amounts of money being deposited, and a guy floating around whom I assume they know to be a member of the Quds Force, I would say it just doesn’t feel right,” said Charles Faddis, a former CIA counterterrorism chief, “beginning with the selection of a target in downtown D.C. It’s so clearly an act of war that it’s hard to imagine why the Iranians would sign on to that. And the tradecraft seems amateurish and sloppy. It’s crazy.”

The chief suspect is Manssor Arbabsiar, 56, an Iranian-American used-car salesman. He has been in the United States since the late 1970s and has been married to two U.S. citizens, both of them Latinas, according to documents and officials. He became a U.S. citizen last year, according to officials.

Some years ago, Arbabsiar befriended a Corpus Christi, Texas, woman whose nephew he believed to be a member of the Zetas cartel, according to officials. The nephew was actually a confidential DEA informant “with direct access to key leadership elements” of the Zetas and the rival Gulf cartel, according to a U.S. law-enforcement official.

Arbabsiar told investigators that he recruited the informant in May at the direction of Arbabsiar’s cousin in Iran, a general in the Quds Force, according to the indictment. The informant advised his DEA handlers, who launched an undercover operation with the FBI. The informant met with the suspect in Mexico to develop the plot, officials say.

Arbabsiar's record includes only minor traffic violations, and he apparently was not a veteran of the drug underworld. Some U.S. officials are puzzled that Iran would deploy an apparently inexperienced operative for a sensitive, potentially disastrous mission.

“I’m getting the impression this is a one-off, opportunistic thing,” said a U.S. national security official who requested anonymity. “From what I’ve seen, this is not a highly trained guy. He had the language, the connections, the ability to travel. His personal situation put him in a position where he was useful. I didn’t get the sense he was a hardened criminal.”

The investigation identified the Iranian general and his deputy, a colonel in the Quds Force who allegedly directed and funded the conspiracy with almost $100,000 wired from a foreign bank to an account set up by the FBI in New York, making the implications grave, officials say. In addition, Treasury Department officials have implicated two top officials of the Revolutionary Guard Corps in the plot. Arbabsiar cooperated after his arrest in late September, allowing the FBI to intercept his phone conversations with the Iranian colonel, Gholam Shakuri, who has been indicted and remains a fugitive.

“The investigators feel there’s a deep connection into the Iranian security establishment,” the national security official said. “The question is: Do you have elements of that apparatus going rogue here? Can you control the Quds Force or smaller entities within it? Or is there a new attempt by the Iranians to come after us?”

Iran has targeted U.S. troops through paramilitary proxies such as Shiite militants in Iraq and the Taliban in Afghanistan, according to counterterrorism experts. In past terrorist attacks, Iranian spies carefully chose locales where weak security forces and an infrastructure of local accomplices enabled them to conceal their tracks, according to Western intelligence officials.

Iran has been blamed for bloody car bombings in Argentina of the Israeli embassy in 1992 and a Jewish community center in 1994, but corruption, ineptitude and political manipulation prevented a complete resolution of those cases. The Quds Force allegedly worked with Hezbollah in those Argentine attacks as well as an aborted bombing in 2008 of the Israeli embassy in Azerbaijan, which neighbors Iran. Azeri police arrested alleged plotters including Iranian diplomats and a veteran Lebanese Hezbollah operative dispatched from South America for the mission, according to Western intelligence officials and the Azeri court charges.

The Iranian plot in Washington allegedly involved discussion of attacks on the Israeli and Saudi embassies in addition to the assassination of the ambassador.

In contrast to operations in nations such as Argentina or Iraq, spectacular attacks on heavily-guarded diplomatic targets in Washington would risk rapid discovery by U.S. authorities and demand major retaliation. Moreover, an alliance between Iranian Islamic extremists and the ruthless capitalists of Mexican cartels for an act of terrorism would be unprecedented — “a monumental leap of faith,” as former DEA official Braun put it.

Western counterterrorism officials say Iranian intelligence has established a growing presence in Latin America, which has large Arab immigrant communities. Iranian and Hezbollah operatives have expanded their roles in crime, fundraising and recruitment in longtime outposts such as Venezuela and the area where the borders of Argentina, Brazil and Paraguay meet. A DEA investigation in 2009 identified a Hezbollah operative in Mexico who had drug connections and access to a stockpile of military arms allegedly stolen from Iraq, according to court documents.

“I believe the Hezbollah-Iranian presence in Latin America constitutes a clear threat to the security of the U.S. homeland,” Roger Noriega, a former top State Department official on Latin American affairs, testified in July at a hearing of the House Homeland Security Subcommittee on Counterterrorism and Intelligence.

But U.S. diplomats downplay the warnings about Iranian and Hezbollah activity in the hemisphere, saying it consists mostly of fundraising. Even officials who take the Iranian threat seriously say they have not seen previous indications of a close partnership between Iranian spies and Mexican drug mafias.

“The main question is: Is there any conceivable way that Mexican DTOs [drug-trafficking organizations] would cooperate with the Quds Force?” a U.S. intelligence official said. “”It doesn’t make good business sense.”

The Zetas of Mexico have massive firepower and formidable systems of communications and intelligence gathering. Mexican cartels have built trafficking networks from Atlanta to Los Angeles; they have been accused of killing U.S. Border Patrol agents, an Immigration and Customs Enforcement agent based in Mexico and people connected to the U.S. consulate in Ciudad Juarez.

In contrast to the bloody turf wars in Mexico, however, Mexican gangsters have largely avoided shootouts and major killings north of the border. They and their corrupt allies in Mexican law enforcement have a keen understanding of a U.S. mentality in which such violence, not to mention links to Islamic terrorism, would trigger a backlash.

The use of Mexican gunmen would certainly make it harder to trace the plot back to Iran. But experts and officials said they doubt that Mexican traffickers would become embroiled with Iran in a kamikaze-like attack that could incite the wrath of the U.S. government. In the wiretapped conversations, the informant posing as a Zetas hit man told Arbabsiar that the planned bombing in Washington might kill many bystanders, including U.S. senators dining at the restaurant frequented by Ambassador Al-Jubeir.

According to the Department of Justice, the Iranian-American allegedly responded: “They want that guy [the ambassador] done [killed]; if the hundred go with him, f**k 'em.”

Arbabsiar apparently did not realize that his interlocutor’s willingness to engage in the slaughter was suspicious. It is harder to believe, however, that chiefs of the crack Quds Force would have been so unsophisticated – or relied on Arbabsiar to hire Mexican triggermen with whom they had not worked in the past, some officials said.

“Assuming they wanted to run an op and have the Mexican cartels take point, they have Hezbollah guys all over South America they have been working through a long time,” said Faddis, the former CIA official. “They have a million other hard-wired ways to get in contact with the cartels.”

A potential explanation: The Iranian-American suspect may have been trying sell the Quds Force on a plan beyond his capacity to execute.

“He may have been trying to bilk them,” Faddis said. “Maybe they got involved to the point of trying to figure out what he can deliver, and it turns out U.S. law enforcement is all over him.”

The Iranian Revolutionary Guard Corps has become a veritable state within a state, Western intelligence officials say. The elite force’s dominance of Iran’s politics and economy has been accompanied by factional infighting, mafia-like moneymaking and sloppy tradecraft. Two IRGC officials were implicated in arms trafficking last year in Nigeria, and one was arrested. The year before, Italian police arrested two alleged Iranian operatives who were wiretapped discussing arms deals and espionage against opposition figures with bosses at the Iranian embassy in Rome and in Tehran.

“When you are dealing with the Iranians, there is often no such thing as the Iranian government but multiple power centers,” Faddis said. “Sometimes they do things that don’t make sense or are contradictory.”