The hack of the Securities and Exchange Commission's corporate filing database likely involved Eastern European criminals who may have been perusing market-moving information stored in the regulator's network for months, according to two people with knowledge of the matter.

It was during a routine maintenance check of the SEC's Edgar system that the agency discovered how long intruders might have had access to company secrets, said one of the people who asked not to be named to discuss findings about the 2016 hack that haven't been disclosed.

Edgar is best known as a massive repository where firms inform investors about everything from their earnings to top executives' share sales. But the aspect of the database that was hacked is largely under the radar and houses test filings that are never meant to be released publicly.