The theft of contact information for job seekers in the database of Monster was greater than the 1.3 million individuals the company reported last week, Chief Executive Sal Iannuzzi said on Wednesday.
While investigating the recent theft, the company learned that its Web site had previously been hacked.
"We're assuming it is a large number. It could easily be in the millions," Iannuzzi said in an interview with Reuters.
The hackers didn't get the kind of information it takes to pull money out of a bank account, according to Monster.com, but the contact data is valuable to criminals who use social engineering techniques to conduct scams.
Contact data provide criminals just enough information to convince some recipients to let their guard down because the use of personal information and other social engineering techniques make the e-mails seem legitimate.