On Friday, as Knight Capital Group's board mulled buyout offers from two suitors, the trading firm took steps toward opening its books for due-diligence reviews that could begin any day, according to someone familiar with the matter.

Getco LLC and Virtu Financial LLC, the two companies interested in buying Knight, signed confidentiality agreements to keep the trading firm's internal books private as part of investments they made in the company in August, said two people familiar with the matter. Informal discussions about an outright purchase of Knight have been going on ever since, and a more official due-diligence process involving a more detailed examination could begin in the coming days, said one of these people, who has knowledge of the deal discussions. (Read More: Knight Capital Up for Sale, May Merge With Rival)