New info on what hackers targeted in massive gov't data breach

China's government denied Friday that it hacked into U.S. government computers, saying it wished the U.S. would trust it more. But cyber security experts say all signs do point to China -- and the hackers stole personal information from more than 4 million current and former federal employees.

A federal law enforcement source tells CBS News the attack against the Office of Personnel Management began last fall.

Experts say it bears similarities to earlier attacks against health care companies Anthem Blue Cross and Premera. All of the hacks appear aimed at personnel records, not financial information.

Jim Lewis, who has advised the government on cyber security for more than a decade, says China is using the attacks to gather intelligence.

"They are collecting huge amounts of data and they are mining it to see if they can find interesting patterns to get a sense of who their opponents are," Lewis told CBS News.

In the wake of high profile attacks like those on Target and Sony, the FBI has stepped up the recruitment and training of new agents in cyber crime.

Assistant Director James Turgal tells CBS News he's looking to hire 700 new specialized cyber agents every year, but finding them is a challenge.

"I'm competing with Fortune 500 companies that can pay twice what I can pay or three times what I can pay ... and I lose those candidates to the private sector and to other government agencies every day," he said.

The FBI has 13,000 agents. With a few years, the bureau would like about 13,000 of them to specialize in cyber crime.