WASHINGTON -- Sen. Al Franken on Tuesday fired off another letter to Uber headquarters in San Francisco, asking the ride-sharing company to further clarify its privacy policies.

This is the second time Minnesota's junior senator has pushed the company to be more transparent about how it stores and uses millions of consumers rider information. 

Late last year, amid news reports Uber threatened to publicize private data about journalists' writing negative stories about the company, Franken wrote the company a letter asking for clarification about its privacy practices. The company responded, though not to Franken's satisfaction, he said Tuesday.

"I appreciate that Uber responded and has expressed its commitment to improving its data privacy and protection policies and practices," he said, in a statement. "However, while I'm pleased that I received a reply, I was, and still am, concerned about the lack of detail in the response."

Tuesday's letter to the company asked for clarification on how Uber defines "legitimate business purpose" when employees are able to access consumer rider information. He also pressed the company on when, and with whom, it shares customer data. 

Uber, which works by consumers sending for drivers via an app on a smart phone, now operates in every major U.S. city and 52 countries worldwide. Last June, its valuation was set at $18 billion. Its smaller competitor Lyft was also pressed by Franken last year to clarify privacy practices. Franken was mostly satisfied with Lyft's response, which arrived earlier this month.

Franken asked Uber to meet a Feb. 11 deadline.