Manchester United are working with the UK's Cyber Security Agency following an attack on their computer systems a week ago which is still causing disruption.
United have not discovered a motive for the attack which was launched on November 20 and said it was not "aware of any breach of personal data associated with our fans and customers".
They have not confirmed whether hackers are demanding a ransom.
United have reiterated that they acted quickly, the attack was stopped and the hackers do not have control of the club's systems.
The investigation has been described as "thorough" and is still ongoing, but United are confident that supporters' data has not been compromised. They are now in the process of restoring systems, although emails are still not operational.
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The National Cyber Security Centre (NCSC) is assisting the team.
A spokesperson said: "The NCSC is aware of an incident affecting Manchester United Football Club and we are working with the organisation and partners to understand impact."
On Thursday, the Premier League side reiterated they were not currently aware of any fan data being affected.
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In a statement, United said: "Following the recent cyber attack on the club, our IT team and external experts secured our networks and have conducted forensic investigations."
The club said it would not comment on who was "responsible for this attack or the motives".
The statement continued: "This attack was by nature disruptive, but we are not currently aware of any fan data being compromised.
"Critical systems required for matches to take place at Old Trafford remained secure and games have gone ahead as normal."
Last week, United informed the Information Commissioner's Office, as required.
They could face a fine from the data regulator if fan data is compromised as a result of the cyber attack.
Manchester United next play in the Premier League at Southampton on Sunday, live on Sky Sports.
