(Associated Press) - Maine Sen. Susan Collins -- a member of the Senate intelligence committee -- says a major breach of computer networks at the government personnel office was carried out by hackers believed to be based in China.

Other officials say the identifying information of at least 4 million federal workers may have been stolen, including people with security clearances.

The Department of Homeland Security said in a statement that data from the Office of Personnel Management and the Interior Department was compromised. The FBI is said to be investigating ``how and why'' the breach occurred.

The Office of Personnel Management is the human resources department for the federal government, and it conducts background checks for security clearances.

Rep. Adam Schiff, the ranking Democrat on the House intelligence committee, called the hack ``shocking.''

The head of a cybersecurity firm says the information stolen could be used to impersonate or blackmail federal employees with access to sensitive information. Workers are being offered credit monitoring and identity theft insurance.