SEC investigations are private -- the evidence is they are issuing subpoenas. From the SEC website:
"All SEC investigations are conducted privately. Facts are developed to the fullest extent possible through informal inquiry, interviewing witnesses, examining brokerage records, reviewing trading data, and other methods. With a formal order of investigation, the Division's staff may compel witnesses by subpoena to testify and produce books, records, and other relevant documents. Following an investigation, SEC staff present their findings to the Commission for its review. The Commission can authorize the staff to file a case in federal court or bring an administrative action. In many cases, the Commission and the party charged decide to settle a matter without trial."
So the subpoena is evidence of an SEC formal investigation.