While she was secretary of state, Hillary Rodham Clinton wrote and sent at least six emails using her private server that contained what government officials now say is classified information, according to thousands of emails released by the State Department.

Although government officials deemed the emails classified after Clinton left office, they could complicate her efforts to move beyond the political fallout from the controversy. They suggest that her role in distributing sensitive material via her private email system went beyond receiving notes written by others, and appears to contradict earlier public statements in which she denied sending or receiving emails containing classified information.

The classified emails, contained in thousands of pages of electronic correspondence that the State Department has released, stood out because of the heavy markings blocking out sentences and, in some cases, entire messages.

The State Department officials who redacted the material cited national security as the reason for blocking it from public view.

Clinton, who is seeking the Democratic presidential nomination, was one of about four dozen State Department officials whose emails were redacted because of national security concerns, according to a Washington Post review. Those officials included top aides such as Jake Sullivan and Cheryl Mills, some of whom would be likely to fill out senior roles in a Clinton administration. All told, 188 of the emails the State Department has released contain classified material.

The extent of the redactions in emails sent by Clinton and others, including ambassadors and career Foreign Service officers, points to a broader pattern that has alarmed intelligence officials in which sensitive information has been circulated on non-secure systems. Another worry is that Clinton aides further spread sensitive information by forwarding government emails to Clinton’s private account.

But it also highlights concerns raised by Clinton and her supporters that identifying classified material can be a confusing process, and well-meaning public officials reviewing the same material could come to different conclusions as to its classification level.

The intelligence community’s inspector general had previously identified four emails out of a sample of 40 that had been sent on her server and contained classified information.