The State Department scrambled over the weekend to secure its unclassified e-mails, shutting down the entire e-mail system after finding evidence suggesting a hacker may have been been poking around.

A senior State Department official said technicians recently detected “activity of concern” in portions of the system handling unclassified e-mail. The official, who spoke on the condition of anonymity to discuss the breach, said that none of the department’s classified systems were compromised.

The shutdown affected the State Department’s unclassified e-mail traffic and access to parts of its public Web site, the official said.

The breach is the latest of a series of electronic intrusions first detected last month on government computer systems at a variety of agencies, from the White House to the U.S. Postal Service to the National Weather Service. The suspected hackers of the White House’s computer network were believed to be working for the Russian government.

The State Department did not seek to publicize that it had been hacked. On Friday, it announced that “maintenance” would be done to the unclassified network during a routine, scheduled outage. But on Sunday, after the Associated Press first reported the breach, officials acknowledged they had found traces of suspicious activity in their system and were updating security in the middle of a scheduled outage. In a sign of how complete the shutdown was, duty officers were using Gmail accounts.

The senior State Department official declined to say how many of the department’s e-mail accounts were affected, or whether personal information on employees may have been exposed. The official said the system is expected to be working normally again “soon.”