More than a quarter of the all-cash luxury home purchases made using shell companies in Manhattan and Miami were flagged as suspicious in a new effort to unearth money laundering in real estate, the Treasury Department said Wednesday. As a result, officials said they would expand the program to other areas across the country.

The expansion of the effort to identify and track the people behind shell companies, begun in March, means that there will now be increased scrutiny of luxury real estate purchases made in cash in all five boroughs of New York City, counties north of Miami, Los Angeles County, San Diego County, the three counties around San Francisco and the county that includes San Antonio.

The examination, known as a geographic targeting order, is part of a broad effort by the federal government to crack down on money laundering and secretive shell companies.

“The information we have obtained from our initial G.T.O.s suggests that we are on the right track,” Jamal El-Hindi, the acting director of the Financial Crimes Enforcement Network within the Treasury, said in a department news release. “By expanding the G.T.O.s to other major cities, we will learn even more about the money laundering risks in the national real estate markets, helping us determine our future regulatory course.”