The bank now says the breach hit more than 360-thousand of its North American credit card accounts.
That's nearly twice as many as Citi initially estimated.
The hackers accessed customers' names, contact information and account numbers on May 24th, but Citi claims no "data critical to commit fraud" was compromised.
The bank has been notifying customers affected by the security breach since early this month.
It has issued new credit cards to more than 200-thousand compromised accounts.