The car company released a on April 20, one week after the intrusion was detected. Jeff Kuhlman, Nissan's head of global communications, said on Thursday that Nissan delayed disclosing the breach sooner in order to cleanse its network of the malicious software and prevent tipping off the hackers.

"We are working with security software specialists and making sure that all the doors are closed and that going forward we have the most secure system we can have," Kuhlman said.

Nissan said in a statement that the malware accessed a data store that held employee user account credentials. Kuhlman said the company is not sure what information the hackers were after.

"As a result of our swift and deliberate actions we believe that our systems are secure and that no customer, employee or program data has been compromised," according to the statement.

Nissan said it would "continue to vigilantly maintain our protection and detection systems and related countermeasures to keep ahead of emerging threats."