A security guard stands at the entrance of United Artists theater during the premiere of the film "The Interview" in Los Angeles, Dec. 11, 2014. Reuters/Kevork Djansezian

It's now known how hackers gained access to Sony's internal network earlier this month, leaking gigabytes of data that have been finding their way around the Internet. The hack was made possible by stealing the login credentials of a Sony system administrator, which grants a user high-level access to company information. Investigators don't think the cyberattack was made possible with inside help.

This new data further drives home the U.S. government's conclusion that North Korean hackers are responsible for the cyberattack. Government officials are expected to officially blame the country for the incident as soon as tomorrow.

Attacks of this sort usually lead to the conclusion that they were perpetuated by someone on the inside, but the U.S. used signals intelligence to follow evidence back to North Korea. There's not yet word of how officials plan to respond to the attack, but one official said, "We do think it's appropriate to respond."

"There is evidence to indicate that we have seen destructive activity with malicious intent that was initiated by a sophisticated actor," said White House Press Secretary Josh Earnest, adding that any response from the U.S. would need to be "proportional." One course of action might include new sanctions against the country, a source told CNN. The most extreme course of action might be to restrict North Korea's dollar-denominated trade by taking action against Chinese banks in business with the country, as was done with Iran and Russia.

A source in law enforcement tells CNN that the investigation in this attack is moving at unparalleled speed as the U.S. prepares to publicly blame North Korea for the attack. The Sony hack is similar to an attack against South Korea last year, and the coding was done in Korean. A source familiar with the investigation told CNN that "the tactics, techniques and procedures were right out of the North Korean playbook."