Equifax Inc. said its systems were struck by a cyberattack that may have affected about 143 million U.S. customers of the credit reporting agency, shedding light on one of the largest and most intrusive breaches in history.

New York’s attorney general opened an investigation into the cyberattack against Equifax Inc., as Connecticut and other states also said they’d seek information about the breach that may have compromised the privacy of more than 140 million U.S. consumers.

After Equifax Inc. revealed that sensitive data on two of every five Americans was exposed in a cyberattack, thousands logged onto a company website to see if they were at risk. For many, the site didn’t work at first. But for those who got through, a nasty surprise was waiting.