By Joseph Menn WASHINGTON (Reuters) - The proliferation of hacking tools known as zero-day exploits is raising concerns at the highest levels in Washington, even as U.S. agencies and defense contractors have become the biggest buyers of such products. White House cybersecurity policy coordinator Michael Daniel said the trend was "very worrisome to us." Asked if U.S. government buying in the offensive market was adding to the problem, Daniel said more study was needed. "There is a lot more work to be done in that space to look at the economic questions... ...