Accounting firm Deloitte says it has launched a thorough investigation into a cyber-attack that hit its email system.
The company said in a statement Monday that the breach had affected "very few clients" and that government authorities were notified.
The Guardian newspaper reported Monday that the breached system had information from a range of clients, including large companies and US government departments.
The newspaper says hackers gained access through an administrator's account last fall and the attack was discovered in March.
Deloitte says no disruption "occurred to client businesses, to Deloitte's ability to continue to serve clients, or to consumers."
The company says it is "deeply committed to ensuring that its cyber-security defenses are best in class, to investing heavily in protecting confidential information and to continually reviewing and enhancing cyber-security."