By Pete Schroeder and Nivedita Balu WASHINGTON (Reuters) -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 that month. 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 also questions why, when Musk did disclose his stake, he used a form meant for passive investors while he was openly questioning Twitter’s policies around free speech…