Sony Pictures Entertainment has told employees companywide to not connect to corporate networks or access email, after the studio was hit Monday by what appeared to be a malicious hacker attack threatening to disclose “secrets,” Variety has confirmed.

The apparent hack was reported earlier by Deadline.com. “We are investigating an I.T. matter,” SPE spokeswoman Jean Guerin said in an emailed statement. The hack apparently is not affecting other divisions of Sony Corp., sources said.

[UPDATE, Nov. 25, 10:30 PT: SPE’s corporate email systems are still offline. “Sony Pictures Entertainment experienced a system disruption, which we are working diligently to resolve,” the company said in a statement.]

According to a source at Sony Pictures, the company is telling employees that the situation may take anywhere from one day to three weeks to resolve. The source said a photo appeared on company computers Monday morning with an image of a skeleton and a message saying “Hacked by #GOP.” The message then says, “Warning: We’ve already warned you, and this is just the beginning… We have obtained all your internal data including secrets and top secrets.”

Sony’s information-technology departments have instructed employees to turn off their computers as well as disable Wi-Fi on all mobile devices.

The “Hacked by #GOP” message warned that the data supposedly obtained from Sony’s systems would be divulged Nov. 24, at 11 p.m. GMT, which is 3 p.m. Pacific/6 p.m. Eastern on Monday. It isn’t clear at this point which individual or group is responsible for the attack, or specifically what the hackers’ aims are.

The SPE attack is being linked to a group calling itself “Guardians of Peace,” Bloomberg reported, citing an anonymous source.

In August, hackers claimed they took down Sony’s PlayStation Network via a denial-of-service attack, which overwhelms systems with bogus network requests. According to the company, no personal data of PlayStation Network’s 53 million users was compromised in the Aug. 24 incident, and access was restored the following day. In 2011, a more serious security breach exposed names and passwords of millions of PlayStation Network customers.