However, experts point out that Medicare's audits don't accurately capture fraudulent claims.

''It's a weak audit that would not detect most fraud,'' said Malcolm K. Sparrow, a professor at Harvard University's John F. Kennedy School of Government and author of the book, License to Steal: How Fraud Bleeds America's Health Care System. ``They only detect claims processing errors, not lies.''

• Government inspections of equipment providers were practically nonexistent until late 2006, when Health and Human Services' Office of Inspector General found almost 500 suppliers were not open for business. Although federal agents shut them down, Medicare allowed 300 of them to reopen after appeals, said Brandt, who called the inspector general's inspections ``cursory.''

Brandt said Medicare now does site inspections more than once a year at every registered equipment supply office in South Florida as well as in Southern California.

Despite such assertions, a Miami Herald reporter accompanied FBI and Health and Human Services agents on unannounced visits to five medical equipment companies in Miami, Hialeah and Doral in May. None was open for business. They were spotted by Sposato, a sleuth-like expert at deciphering code-heavy Medicare claims. In April, she zeroed in on extraordinarily high bills submitted by the Hialeah business and the four others on her computer spreadsheets.

But the FBI is narrowing its focus on only one of those five businesses -- a storefront in a strip center off Calle Ocho -- because the rest didn't receive much in Medicare reimbursements. The Little Havana company, however, filed about $200,000 in false medical equipment claims in March, and Medicare paid it $91,000, according to the FBI.

In her 12 years at the U.S. attorney's office, Sposato said the worst Medicare abusers have been the medical equipment suppliers and HIV infusion clinics billing for bogus services.

But now some in South Florida are branching out into other areas of fraud, including home healthcare assistance and rehabilitation facilities, and into other Florida regions, such as Orlando, Tampa and St. Petersburg.

''The fraud never stops,'' Sposato said. ``Every time you build a better mousetrap, the mouse gets smarter.''