Sources in the investigation told the Boston Herald federal and state authorities are looking at whether police in Brockton, Mass., ignored a report of prostitution from a worker at one of the spas.
The source said investigators were at the Brockton police station Wednesday, the newspaper reported.
A suspended state trooper is also a focus of the probe, law enforcement sources said.
Spas in Canton, Brockton and Norwood were raided last week, and prosecutors allege masseuses were providing sex for cash and selling drugs.
Charges have been filed against the owners of the spas, 42-year-old Terry Mussari and 31-year-old Tiffany Burgess, authorities said.