A federal judge on Friday struck down a law that allows the Federal Bureau of Investigation to secretly seize subscriber information from Internet companies and other communications service providers without a court warrant. The ruling, by Judge Susan Illston of Federal District Court in San Francisco, struck down the law because it forbids companies from disclosing that they had received such an order, called a National Security Letter, making the statute “impermissibly overbroad” under the First Amendment, she wrote. She also struck down a statute on legal challenges by recipients of the security letters, but stayed her ruling to allow the government to appeal.