These firms have received two-page letters from the Justice Department asking some pointed questions about their business: Kohlberg Kravis Roberts, Carlyle Group, Silver Lake Partners and Clayton, Dubilier & Rice. Prominent firms that did not receive the letter--not yet anyway--include: Texas Pacific, Blackstone Group, Bain Capital and Thomas H. Lee Partners. Still, the concern is palpable in the industry. Many lawyers have been hired. This may amount to nothing, but it underscores the fact that the industry does have some regulatory concerns, and many are in favor of forming some sort of trade association in Washington. Nascent steps are underway. We'll likely hear more about this soon.

For more on this:
- Here's a New York Times article
- More on the probe. Article