Shares in Equifax plunged 16 per cent as US markets opened after revelations the credit report behemoth had been hit by a massive cyber breach.
Equifax admitted hackers had obtained social security details as well as dates of birth and addresses.
The company revealed details of the breach after the bell, sending shares flying in after-hours trades. The firm's shares fell as much as 19 per cent in unofficial trading.
The Atlanta-based firm said it discovered the breach in late July. Criminals took advantage of a weakness in a US website application and accessed data.
Equifax said it was working with law enforcement agencies, while the FBI is tracking the situation, a spokesperson told Reuters.
Cyber-security experts have been drafted in to investigate the breach and their analysis is “substantially complete” and will be concluded within weeks.
Equifax chief executive Richard Smith said the incident "strikes at the heart of who we are and what we do".
He said: "I apologise to consumers and our business customers for the concern and frustration this causes.
"We pride ourselves on being a leader in managing and protecting data, and we are conducting a thorough review of our overall security operations."