The full extent of the massive digital spying operation that has been undermining U.S. government systems for months may never be known. But the Treasury Department has managed to established that suspected Russian hackers broke into email accounts and breached the office that houses its top officials, according to a senior senator. Sen. Ron Wyden, the top Democrat on the Senate Finance Committee, told The Wall Street Journal that the Treasury doesn’t know what information was stolen, but confirmed systems had been compromised. “According to Treasury staff, the agency suffered a serious breach, beginning in July, the full depth of which isn’t known,” Wyden said. The suspected Russian operation is believed to be one of the worst ever hacks of federal computer systems.