For Target, the Breach Numbers Grow

The stunning figure represents about a third of all American adults at the low end, and is nearly three times as great as the company’s original estimate at the upper end. The theft is one of the largest ever of retail data.

Not only did Target’s announcement disclose a vastly expanded universe of victims, but it revealed that the hackers had stolen a broader trove of data than originally reported. The company now says that other kinds of information were taken, including mailing and email addresses, phone numbers or names, the kind of data routinely collected from customers during interactions like shopping online or volunteering a phone number when using a call center.

On Dec. 19, Target confirmed reports that payment data was stolen from about 40 million customers who shopped in its stores in the United States from Nov. 27 to mid-December. As its investigation into the theft continued, the company said it had found that an additional quantity of data, collected over time on 70 million people and stored separately from the in-store data, was stolen.

The latest subset of potential victims includes customers who may not have shopped at Target during the holiday period.

Although there is probably some overlap between the two groups, the company said it did not know the extent.

When Target’s security breach became public in mid-December, customers flooded help lines, the company’s website and its Facebook page expressing worry and irritation. And it now appears that wary customers steered clear of Target stores during the last days of the shopping season, as suggested by the company’s statement on Friday that sales declined noticeably after the disclosure.

The effect of the data theft has reached far beyond one of the nation’s largest retailers. Major credit card companies and banks have been issuing warnings about potential fraud to their customers and providing them with new cards and account numbers as a precaution. Some banks have limited cash withdrawals. As banks and companies continue to monitor customers’ accounts for suspicious activity, the Secret Service and the Justice Department have opened an investigation.

“This will impact many Target business partners — Visa, MasterCard and the host of banks and credit agencies that now have to keep an eye on the 110 million customers now vulnerable to identity theft,” said Hemu Nigam, founder of SSP Blue, a security and privacy consulting firm. “It affects more than Target customers. It affects mortgage lenders and car sales. It affects the entire economic infrastructure.”

Fraud experts said the information stolen from Target’s systems quickly flooded the black market. On Dec. 11, shortly after hackers first breached Target, Easy Solutions, a company that tracks fraud, noticed a 10 to twentyfold increase in the number of high-value stolen cards on black market websites, from nearly every bank and credit union.

The company apologized again on Friday for the broadening violation of its customers’ privacy.

“I know that it is frustrating for our guests to learn that this information was taken, and we are truly sorry they are having to endure this,” Gregg W. Steinhafel, Target’s chief executive, said in a statement.

Security experts say that clever hackers could potentially piece together customers’ stolen information for identity theft or for use in a so-called spear phishing attack, in which hackers send a highly tailored emails to victims asking them to click on a link or download an attachment that, once opened, gives hackers a foothold into their computers and employers’ networks.

Target has been working with a forensics team at Verizon, and it has also consulted with Mandiant, a security firm specializing in data breach recovery, which recently agreed to be acquired by FireEye, the security software company, for close to $1.1 billion.