The Justice Department under President Trump secretly obtained a grand-jury subpoena last year in an attempt to identify the person behind a Twitter account dedicated to mocking Representative Devin Nunes of California, according to a newly unsealed court document. But Twitter fought the subpoena, as well as an associated gag order barring the company from talking about it publicly. Twitter executives raised skepticism about whether the Justice Department might be abusing federal criminal law-enforcement power to retaliate against a critic of Mr. Nunes, a Republican who is a close ally of Mr. Trump, in violation of the First Amendment. Ultimately, according to a person familiar with the matter, the Justice Department withdrew the subpoena this spring, after President Biden took office. What was going on behind the subpoena remains murky.