The department used its most intense investigative methods on Spitzer, such as reviewing his financial documents, listening to his phone calls and following him while looking into his involvement with the Emperor's Club prostitution ring, The New York Times reported Friday.

Methods used in the Spitzer case appeared to be more intense than typical prostitution investigations by the Justice Department, the Times report said. Federal prosecutors usually go easier on prostitution criminals, and fail to act unless millions of dollars, child exploitation or human trafficking are involved, it said.

Government officials have justified the depth of the investigation, saying it was necessary because Spitzer was the top elected official in New York.

"If they've got some evidence of a high-ranking public official involved in violations of federal criminal code, it may not be unreasonable for them to pursue it," Justice Department lawyer Bradley D. Simon said.