When American Airlines recently admitted violating federal laws that govern the storage of hazardous materials and agreed to pay an $8 million fine, the action wasn’t prompted by the prime enforcer of airline safety rules, the Federal Aviation Administration.

The outcome was part of a rare criminal case brought by South Florida’s U.S. attorney’s office against the country’s No. 2 airline. The move marked a new focus for the office, which in some ways is moving into a role historically preserved for the FAA — that of airline safety regulator.