The U.S. Securities and Exchange Commission has launched a formal investigation into the finances of Miami-based Jackson Health System, focusing on a multimillion dollar bond sale last year. Federal investigators are seeking reams of accounting documents, financial projections, and internal e-mails that show how and why the public hospital lost $244 million last year. At issue is an $83.3 million bond sale last September by Miami-Dade County to raise money for capital improvements at Jackson that include upgrading fire alarms and renovating elevators.