In the 2180s, there were several private companies that supplied EarthGov with sleeper drugs, and Waters Corporation was one of them. They had plants across the US, including one in Seattle. Waters corporation also produced other pharmaceutical products, and one of their chief competitors was Rentech Corporation.
In 2188, in exchange for Rentech's assistance to the Underground in various ways, its leaders (the Dexters and their associate, Walters) agreed to bomb one of the Waters' Corporation chemical plants. ("Because we're terrorists. That's what we do.")
They and their people didn't have much trouble sneaking in and wiring the building with explosives - as rogue telepaths, they could easily make people not see them. Canon's not clear how they got around the security systems, but somehow, they did.
Then Fiona Dexter crashed a party where Mr. Waters, the company founder and CEO, was celebrating, pretended to be his illegitimate daughter, got him alone, shot him with a tranquilizer, scanned him, and left him unconscious with "Live Free" written across his face in marker. When he woke, he knew that his Seattle plant was about to be bombed, and immediately alerted company security.
The rogue telepaths waited for the plant to be evacuated, and then detonated the whole place. One bomb blew up the building itself, and then a second blew up the chemical storage tanks. aiming for maximal dispersal of the substances inside. (That the newly-evacuated plant workers were still at ground zero of the chemical blast never mattered to the telepath terrorists, who considered themselves to be entirely humane because they gave the company a warning.)
Nor did they care about the normals in the nearby city - in fact, the plan all along was to poison them. They triumphantly sped off in a pickup truck, glad the wind was holding and the cloud of fumes would be carried directly into the city.
The tanks that they detonated contained the substance that was then used as a telepathy suppressant. By sending a plume of this chemical over the city, the rogues hoped to poison normals, and thereby show them that the stuff was dangerous and telepaths should never be injected with it.
Well, thousands did get sick, with nausea, vertigo, and other ailments. People lost control of their vehicles and traffic accidents occurred. Mass panic broke out in the city as people tried to flee, jamming up the highways even more - and this made it even harder for emergency vehicles to get into the city and transport people to hospitals. The hospitals, meanwhile, were entirely unequipped to handle a mass-casualty attack on the city, and the medical staff were as poisoned as everyone else.
The rogues were just happy that the chaos covered their escape to a private (Rentech) airstrip, so they could make their getaway to New Orleans.
People in Seattle and the surrounding towns were sick for a week. Lots of people died, if not from the drugs themselves than from vehicular accidents, lack of food (people got shut in at home or work with whatever they had on hand), contaminated water, or the complete shutdown of routine medical care in the city.
Matthew and Fiona Dexter had sex on a private jet and celebrated their victory.
But the attack on Waters' plant didn't have its intended effect. (Of course not.) Normals didn't say, "oh, wow, these drugs are so evil, we'd better abolish the Corps and let telepaths be free!" If anything, they were more angry than ever both at rogue telepaths and at the Corps' seeming ineffectiveness at addressing the growing problem of rogue telepath terrorism. Yes, there was a push to replace the then-used telepathy suppressant drug with a safer alternative (which did eventually come to pass... the "safer alternative" is what Ivanova's mom was on, so we're clear), but that was it. At the time, there was even a push to abolish telepathy suppressants as an option entirely (telepaths would be given only the options of joining the Corps or going to prison, nothing else), but that idea lost steam. They didn't accomplish much: the drugs were altered so that a similar attack would no longer be able to paralyze a city, but that change would have happened anyway, since the new formula also made the drug companies more profit.
Fiona Dexter felt so proud of what they'd done. She felt so good when they kidnapped kids or blew up a sleepers factory. But she needed more in life, so she decided to have a baby.