DETROIT (AP) — The U.S. attorney's office on Thursday announced it has charged 28 people with mortgage fraud in 15 separate criminal cases.

The offenses, which led to losses of more than $50 million, include wire, mail and bank fraud, money laundering and conspiracy. The suspects are from the Detroit and Flint areas, and include real estate appraisers, investors, mortgage brokers and lenders.

The U.S. attorney's Detroit office said in a statement it has been filing the complaints since March and is working with the FBI, the U.S. Postal Inspection Service and other federal agencies and departments. Some of those charged have already pleaded guilty.

FBI spokeswoman Sandra Berchtold said ongoing investigations could lead to further charges.

The Michigan efforts are part of a national crackdown. Since March 1, 406 people have been arrested nationally in the sting dubbed "Operation Malicious Mortgage" resulting from 144 cases across the country.

Law enforcement officials said their stepped-up focus on mortgage cases aims to combat problems that have grown out of the risky lending practices prevalent until the mortgage market collapse started last year.

Officials have identified 10 "mortgage fraud hotspots" nationwide in California, Colorado, Texas, Minnesota, Michigan, Illinois, Ohio, New York, Georgia and Florida.

Earlier this year, the Mortgage Asset Research Institute said Florida led the nation in mortgage fraud in 2007 for the second straight year, followed by Nevada, Michigan, California, Utah and Georgia. The institute is a subsidiary of Alpharetta, Ga.-based technology and information services company ChoicePoint Inc.