The U.S. homeland security chief says investigators have collected "strong evidence" leading to a prime suspect in the massive cybersecurity breach of government records that compromised personal information about millions of federal workers and people applying for national security clearances.

Homeland Security Secretary Jeh Johnson said Wednesday the government is not ready to officially disclose who it thinks was behind the attack on its computers earlier this year, although Director of National Intelligence James Clapper, has said Chinese state interests are "the leading suspect."

In two hacks into Office of Personal Management (OPM) computers, personal information about 4.2 million current and former government workers was stolen, as well as millions of records about people seeking government security clearances.

Johnson told a Washington public policy research group, the Center for Strategic and International Studies, the government is making a concerted effort to thwart new attacks on its computers. He said the U.S. has significantly increased the scope of its monitoring throughout the government but not to the degree it wants.

"To be frank, our federal cybersecurity is not where it needs to be," he said.

He said cyberattacks against the government and American business interests were widespread.

"These threats come from a range of actors, including nation-states with highly sophisticated capabilities, profit-motivated criminals and ideologically motivated hackers or extremists," Johnson said.