Nobody pretends that marketing a product in a capitalist economy is a completely honest process. The entire enterprise can be captured in the caveat, “Your results may vary.” But while companies are permitted all manner of advertising puffery, there is one fundamental rule they’re expected to obey: Don’t make stuff up.

It’s the reason that while soda manufacturers aren’t yet required to warn consumers that the use of their product can lead to obesity, diabetes and more, they’re still not allowed to run ads saying that, whattya’ know, according to scientific research, drinking a 32-ounce soda containing the equivalent of 16 packets of sugar is a perfectly healthy thing to do. It’s the reason that while manufacturers of disposable diapers may plausibly make the case that laundering cloth diapers actually has a bigger environmental impact than throwing away paper ones, they don’t claim that a landfill stuffed with dirty nappies is actually good for the planet.

And it’s the reason too that a newly announced coalition of attorneys general from 25 states and cities as well as the Virgin Islands—led by New York State Attorney General Eric Schneiderman—is right to be investigating the oil companies and their industry groups to determine if they deliberately misled consumers and shareholders about the risks of climate change. It was Schneiderman alone who launched the investigation last November, looking into whether ExxonMobil knew about the risks of greenhouse gases as long ago as the early 1980s and took prudent steps to protect its own assets—such as raising its offshore oil platforms to prepare for rising sea levels—at the same time it was bankrolling efforts to deny there was any meaningful danger at all.

In 1996, for example, according to a joint investigation by the Columbia University School of Journalism and the Los Angeles Times, engineers at Mobil Oil—which had yet to merge with Exxon—were drafting design specifications for a gas field project off the coast of Nova Scotia and made it a point to note, “An estimated rise in water level, due to global warming, of 0.5 meters,” was possible over the course of the project’s 25 years.

But a newspaper ad the company took out the next year had a decidedly different tone: “Let’s face it: The science of climate change is too uncertain to mandate a plan of action that could plunge economies into turmoil. Scientists cannot predict with certainty if temperatures will increase, by how much and where changes will occur.”

That’s a pattern that goes back a lot of years. As long ago as the early 1980s, Exxon and other oil companies were investigating the role of greenhouse gasses in warming the planet and were doing solid, if secretive, research—research that Schneiderman himself applauds. “They were compared to Bell Labs as being at the leadership of doing good scientific work,” he said in an interview with PBS’s Frontline.

Good science being done with one hand, however, was being offset by faux science spread by the other. The companies bankrolled groups like the vaguely named Global Climate Coalition and the Frontiers of Freedom Institute that conducted lobbying and advertising campaigns to cast doubt on climate research, framing the entire matter as just too complex and uncertain to warrant action.

A leaked memo of a 1998 meeting of the American Petroleum Institute, the industry’s trade association, is especially damning. At the time, the group was working to develop a response to the 1997 Kyoto Protocol, a global effort to cut greenhouse emissions.

Repeatedly stressing the need to highlight the “uncertainties” of climate science, the memo argued: “Unless ‘climate change’ becomes a non-issue, meaning that the Kyoto protocol is defeated and that there are no further initiatives to thwart the threat of climate change, there may be no moment when we can declare victory for our efforts…Victory will be achieved when average citizens ‘understand’ (recognize) uncertainties in climate science…'”

If that sounds like a page torn from the pernicious playbook of the tobacco industry, it’s because it is. In 1958, the manufacturers who brought you “More Doctors Smoke Camels Than Any Other Cigarette” and Lucky Strike’s “I protect my voice with Luckies” established The Tobacco Institute, a group charged with promoting both ostensible good news about cigarettes and uncertainty about health studies that said otherwise. That kind of back and forth was more than enough doubt to keep a lot of smokers smoking—and ultimately suffering for it.

Assuming the oil industry has indeed been practicing that kind of flim flam—and so far Schneiderman’s investigation is only an investigation—it’s not only unethical but possibly illegal. Knowing the clear and present danger of climate change but taking out ads that argue the opposite could be a violation of New York State’s Martin Act, which outlaws fraud or misrepresentation in the sale of securities. You’re free to sell your stock, but not to fib about the outlook for the company’s future. In 2015, Sen. Sheldon Whitehouse, (D, RI) even raised the specter of turning to the RICO (Racketeer Influenced Corrupt Organization) Act, to call the oil companies to account—the same law used against Big Tobacco.

The oil industry denies ill-intent, and points—rightly—to the fact it quit funding most climate-denial groups about ten years ago. And even Schneiderman concedes that there’s a difference between corporate advocating and fabricating—that oil companies can certainly argue their side without lying about the other.

Honest advocacy may indeed be all the companies were practicing. But if the tobacco lesson teaches us anything, it’s that corporate interests can sometimes speak a lot louder than scientific facts. And when they do, people get hurt.

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Write to Jeffrey Kluger at jeffrey.kluger@time.com.