Well, if it can be shown that they lied or omitted material information in the course of soliciting equity investments, that's a Rule 10b-5 violation, the SEC's big club. The fact that Theranos is a private company is immaterial, as is the possibility that none of its investors may have complained.
Theranos has at least two mainstream venture funds among its investors. These funds have limited partners, most of which are fairly famous institutions (including big-name universities). These LPs hate to see their money squandered -- and they may feel a legal obligation to raise a ruckus.
