The U.S. Securities and Exchange Commission (SEC) is looking into Tesla Chief Executive Officer Elon Musk’s disclosure of his stake in Twitter Inc. in early April, according to a letter the agency sent to him in April.

In the letter, now made public by the SEC, the regulator asks Musk why it appears he did not file required paperwork within 10 days of the acquisition, and to provide more information on his public statements on the platform regarding whether Twitter adheres to free speech principles.

Specifically, the SEC asked Musk to explain why he opted to ultimately file a “13G” disclosure form, which is meant for investors who plan to hold their shares passively instead of a “13D” form, which is for activist investors who intend influence management and policies of the company.