WASHINGTON — So now maybe every Quds Force operative or al-Qaida sleeper agent will have to worry that when he discusses a plot, he may unwittingly be talking to an undercover informant from the DEA or the FBI. And that paranoia is certainly all to the good.

But I have a nagging concern about the way the United States keeps “stinging” potential domestic terrorists. These concerns have grown stronger in the week after the noisy indictment of Mansour Arbabsiar, the Iranian-American used-car salesman who allegedly wanted to blow up a restaurant to kill the Saudi ambassador to Washington.

I don’t doubt that the Iranians are capable of such a high-risk attack, or that the alleged conspirators made the statements quoted in the indictment. It’s a scary case, all the more so for the haphazard process in which a Quds Force colonel in Tehran, in a recorded phone call, appears to approve a deadly bombing with a bland code phrase about buying a Chevrolet.  

What’s troubling about this case is that, like so many other domestic terrorism prosecutions, it surfaced because of a lucky accident. Arbabsiar allegedly sought to impress Quds Force leaders in Tehran with his connections to the Mexican drug mafia, and in the process blundered into an informant from the Drug Enforcement Administration who was working in the Zetas cartel.

Arbabsiar strikes me, the more I read the improbable details of his indictment, as what intelligence officers sometimes call a “peddler.” That is, someone who’s down on his luck, trying to hustle some business as a freelance plotter. These people are like small-time hoodlums, hoping to score by trading bits of intelligence that come their way, or, in this instance, volunteering to organize a hit on someone they know the boss would like to see dead.

It’s frightening that Iran’s supposedly elite Quds Force would work with such an erratic character. But oddballs like Arbabsiar have a way of surfacing in domestic terrorism cases. Consider some recent examples, suggested by University of Texas law professor Bobby Chesney; they illustrate the unlikely fish in the trawlers’ net:

• Rezwan Ferdaus, an American of Bangladeshi descent was arrested Sept. 28 after allegedly bragging to undercover FBI agents that he planned to fly model aircraft filled with explosives into the Pentagon and the Capitol.

• Agron Hasbajrami, an Albanian immigrant, was picked up Sept. 6 after the FBI intercepted his messages supporting terrorist groups in Pakistan; an FBI informant offered to help him join a radical group overseas, and he was arrested at JFK airport before boarding a plane to Turkey.

• Laguerre Payen, a Haitian immigrant, was sentenced to 25 years last month for plotting with an undercover agent to shoot what were actually fake missiles at synagogues and military airplanes in the New York area in 2009.

• Siavosh Henareh, a Romanian, and three other conspirators were arrested in July after a DEA undercover operation in which they discussed smuggling heroin into the United States and using the sales proceeds to buy Stinger missiles and AK-47 rifles for Hezbollah.

And that’s just from the past few months. Similar examples include a 2006 prosecution of seven members of a Miami religious cult who conspired with phony al-Qaida operatives; a 2008 conviction of a Syrian who agreed to sell weapons to fake Colombian terrorists; and the 2005 conviction of a rice trader who tried to sell a surface-to-air missile to an FBI agent pretending to be a Somali terrorist.

Kudos to the FBI and DEA for clever operations; these cases, near as I can tell, weren’t entrapment but good, aggressive law enforcement.

But it would be better to learn about plots because you have penetrated to the core of your adversary, rather than by picking up a stray operative or wannabe at the periphery. And it’s important not to confuse such peripheral figures with the committed plotters who took months to organize the Sept. 11, 2001, attacks.

The most dangerous conspirators are the ones who won’t fall into the traps — who won’t talk to a potential informer, who won’t discuss operations over an open phone line, who won’t swim so eagerly into the net. U.S. law-enforcement stings will be good at catching sloppy terrorists, but we shouldn’t confuse these successes with the deep and long-running intelligence operations that can snag the careful ones.

And finally, if the Quds Force will talk to a loser like Arbabsiar, who else might they have on the line?

David Ignatius’ email address is: davidignatius@washpost.com.