A federal panel said Wednesday that it had issued a subpoena to the credit-rating agency Moody’s Investors Service after the firm failed to promptly respond to its request for documents and e-mail messages. The subpoena suggested an intensified activity by the bipartisan panel, known as the Financial Crisis Inquiry Commission, which was created to examine the causes of the financial crisis. It was the first such subpoena issued by the panel, which was created by Congress last year and is required to complete its findings by Dec. 15. The panel has been criticized for getting off to a slow start and being unfocused in its inquiry, given the wide scope of its mandate.