Google acknowledged last Friday that its Street View cars around the world had inadvertently collected more personal data than previously disclosed, including complete emails and passwords.
But Google assured the FTC that it had not used and would not use the collected data in any product, the letter said.
"Because of these commitments, we are ending our inquiry into this matter at this time," wrote David Vladek, director of the FTC's Bureau of Consumer Protection.
Vladek also asked that Google continue its dialogue on consumer privacy.
Google had no immediate reaction.
Regulators in Britain, France, Germany, Italy and Spain, among others, have opened investigations into the matter.
The announcement comes just days after Canada's privacy watchdog said Google had collected complete emails and accused Google of violating the rights of thousands of Canadians.
Connecticut Attorney General Richard Blumenthal, who leads a coalition of more than 30 states, said in a statement on Friday that Google's disclosure about the types of data it collected "validates and heightens our significant concerns." He noted then that the investigation was continuing.