Sony Pictures Entertainment is slowly digging itself out after a computer hack buried the studio’s corporate network last week.

The company is still reeling from the disruption, but progress has been made. Certain critical business systems are back up and running, and email is expected to return on Monday, according to an individual with knowledge of the situation.

The hacking caused five films, including unreleased movies such as “Annie” and “Mr. Turner,” to leak online, but the company has not figured out how the breach took place. It is working with law enforcement officials to investigate the cause of the security failure and has hired security firm FireEye and its Mandiant unit to look into the incident.

“Sony Pictures continues to work through issues related to what was clearly a cyber attack last week,” a company spokeswoman said in a statement to Variety. “The company has restored a number of important services to ensure ongoing business continuity and is working closely with law enforcement officials to investigate the matter.”

Sony has not had email and other electronic systems running since last Monday, when a group calling itself “Guardians of Peace” took credit for the attacks. At the time, Sony’s computer system displayed an image of a skeleton along with a message that read, “Hacked by #GOP.” A news report on Re/Code last Friday speculated that the hack might have been orchestrated in conjunction with North Korean officials as punishment for the studio’s involvement in “The Interview.” The James Franco- Seth Rogen comedy centers on a talk show host tasked with assassinating North Korean dictator Kim Jong-un.

The North Korean link is one of many possible scenarios being investigated, according to an insider.