WASHINGTON – In more than a dozen classified rulings, the nation’s surveillance court has created a secret body of law giving the National Security Agency the power to amass vast collections of data on Americans while pursuing not only terrorism suspects, but also people possibly involved in nuclear proliferation, espionage and cyberattacks, officials say.

The rulings, some nearly 100 pages long, reveal that the court has taken on a much more expansive role by regularly assessing broad constitutional questions and establishing important judicial precedents, with almost no public scrutiny, according to current and former officials familiar with the court’s classified decisions.

The 11-member Foreign Intelligence Surveillance Court, known as the FISA court, was once mostly focused on approving case-by-case wiretapping orders. But since major changes in legislation and greater judicial oversight of intelligence operations were instituted six years ago, it has quietly become almost a parallel Supreme Court, serving as the ultimate arbiter on surveillance issues and delivering opinions that will most likely shape intelligence practices for years to come, the officials said.

Last month, a former National Security Agency contractor, Edward J. Snowden, leaked a classified order from the FISA court, which authorized the collection of all phone-tracing data from Verizon […]