A special counsel investigation into allegations of collusion between President Trump’s campaign and Russians could end up exposing illegal activity from lobbyists and consultants at some of Washington’s most powerful firms across the political spectrum.

At least three major lobbying firms have already been identified or had their work described in court documents laying out the criminal charges against two of Trump’s former campaign aides. Those former associates — Paul Manafort, who worked on Trump’s campaign between March and August 2016, and Rick Gates, Manafort’s deputy — face a 12-count indictment related to the false foreign lobbying disclosure forms they filed after years of failing to register their activities, as well as their efforts to launder the millions of dollars they earned from their undisclosed lobbying.