The Chinese government hacked a noncritical Defense Department computer system in June, a Pentagon source told FOX News on Tuesday.

Pentagon investigators could not definitively link the cyber attack to the Chinese military, the source said, but the technology was sophisticated enough that it indicated to Pentagon officials — as well as those in charge of computer security — that it came from within the Chinese government.

The source's information directly contradicts Chinese claims earlier Tuesday, in which officials called the allegations "groundless." The Chinese government, officials said, opposes cyber crime.

"It's a safe assumption that the technology was resident in the state," the source told FOX. It was a "complicated attack."

"These hacking attacks go on everyday but this was a more complicated attack with more sophisticated technology that broke through the current firewalls," the Pentagon source said. "It's a constant game of cat and mouse. This was a wake-up call for us."

The source added that the area hacked was not a very important one. It was a section of the office of the Defense secretary that deals with policy and administrative matters comprised of all unclassified e-mail accounts. The source added that the area is not significant to any operational security.

The Pentagon computers were taken off line for about 3 weeks, according to Pentagon spokesman Bryan Whitman.

The Pentagon is also reviewing its policy on BlackBerry devices given the threat of cyber attacks. Officials said Tuesday that the military is pursuing a sophisticated way to protect the handy wireless devices, and that the department is likely going to be expanding their use.

The Financial Times, citing unnamed officials, reported Monday that the People's Liberation Army hacked into a computer system in the office of Defense Secretary Robert Gates in June. The attack forced officials to take down the network for more than a week, the report said.

Click here to read the full story in The Financial Times.

China denied the reports.

"Some people make groundless accusations against China" that its military attacked the Pentagon, Chinese Foreign Ministry spokeswoman Jiang Yu said at a regular news briefing.

"China has all along been opposed to and forbids criminal activities undermining computer networks, including hacking," she said. "China is ready to strengthen cooperation with other countries, including the U.S., in countering Internet crimes."

It was the second time in two weeks that China was accused of hacking into a foreign government's computers. On the eve of German Chancellor Angela Merkel's visit to Beijing last week, the weekly Der Spiegel said computers at the Chancellery and three ministries had been infected with so-called Trojans, or spy programs.

The report, which did not specify its sources, said Germany's domestic intelligence agency believed a group of hackers associated with the People's Liberation Army might have been behind the alleged hacking.

At the time, Premier Wen Jiabao called the reported hacking a "matter of grave concern" and said China would take "firm and effective action" to prevent such crimes.

The Pentagon warned earlier this year that China's army is emphasizing hacking as an offensive weapon. It cited Chinese military exercises in 2005 that included hacking "primarily in first strikes against enemy networks."

There were news reports in July that the State Department was recovering from large-scale network break-ins affecting operations worldwide. The hackers appeared to target the department headquarters and offices dealing with China and North Korea, it was reported.

However, analysts have said that China is home to a large number of insecure computers and networks that hackers in other countries could use to disguise their locations and launch attacks.

FOX News' Jennifer Griffin and The Associated Press contributed to this report.