By Dustin Volz and Lisa Lambert WASHINGTON (Reuters) - Six senators on Tuesday demanded that Yahoo Inc explain why hackers' theft of user information for 500 million accounts two years ago came to light only last week and called the company's handling of the breach "unacceptable." The lawmakers, all Democrats, said they were "disturbed" that the 2014 intrusion, which was disclosed by the company on Thursday, was detected so long after it occurred. "That means millions of Americans’ data may have been compromised for two years," the senators wrote in a letter to Yahoo Chief Executive Marissa Mayer. "This is unacceptable." A Yahoo spokesman said the company would respond in a "timely and appropriate manner" to the letter, which was signed by Senators Patrick Leahy, Al Franken, Elizabeth Warren, Richard Blumenthal, Ron Wyden and Edward Markey.