WASHINGTON — China-based hackers are suspected once again of breaking into U.S. government computer networks, and the entire federal workforce could be at risk this time.

The Department of Homeland Security said in a statement data from the Office of Personnel Management — the human resources department for the federal government — and the Interior Department had been compromised.

“The FBI is conducting an investigation to identify how and why this occurred,” the statement said.

The hackers were believed to be based in China, said Sen. Susan Collins, a Maine Republican.

Collins, a member of the Senate Intelligence Committee, said the breach was “yet another indication of a foreign power probing successfully and focusing on what appears to be data that would identify people with security clearances.”

But in Beijing, the Chinese Foreign Ministry dismissed the allegations.

A spokesman for the ministry, Hong Lei, said at a regular news briefing that Beijing hopes the U.S. would be “less suspicious and stop making any unverified allegations, but show more trust and participate more in cooperation.”

Beijing routinely dismisses any allegation of its official involvement in cyberattacks on foreign targets, while invariably noting China is itself the target of hacking attacks and calling for greater international cooperation in combating hacking.

“We know that hacker attacks are conducted anonymously, across nations, and that it is hard to track the source,” Hong said. “It’s irresponsible and unscientific to make conjectural, trumped-up allegations without deep investigation.”

A U.S. official, who declined to be named because he was not authorized to publicly discuss the data breach, said the breach could potentially affect every federal agency. One key question is whether intelligence agency employee information was stolen. Former government employees are affected as well.

The Office of Personnel Management conducts more than 90 percent of federal background investigations, according to its website.

The agency said it is offering credit monitoring and identity theft insurance for 18 months to individuals potentially affected. The National Treasury Employees Union, which represents workers in 31 federal agencies, said it is encouraging members to sign up for the monitoring.