By Paul Ziobro 

Target Corp. is re-examining why its data-security team missed signs that hackers were inside its system and has plugged some critical security gaps that helped enable one of the largest credit-card thefts in corporate history.

In an update ahead of a congressional hearing, Target said the hackers appeared to have first entered its system on Nov. 12, 2013, more than a month before the discount retail chain's investigators concluded that a breach had occurred. The company's security system had logged suspicious activity in the interim, but the company decided not to follow up after it was evaluated by its security team.

"With the benefit of hindsight and new information, we are now asking hard questions regarding the judgments that were made at that time and assessing whether different judgments may have led to different outcomes, " according to prepared remarks Target Chief Financial Officer John Mulligan plans to deliver Wednesday afternoon to the Senate Commerce Committee.

Mr. Mulligan is set to make his third appearance in the last two months on Capitol Hill, where legislators are holding hearings about the events that led to the data breach. Lawmakers also are considering legislation to regulate how quickly consumers are notified that their data were compromised and whether an entity such as the Federal Trade Commission should set security standards for retailers.

Target said it since has taken steps to beef up its data security, such as building tougher walls between parts of its network and adding more "two-factor" authentication protocols so that hackers can't just log in using a stolen password. Those two lapses were criticized by data-security professionals as basic safeguards that companies should take to keep their networks secure.

Target's data breach, in which 40 million credit- and debit-card numbers were stolen in the weeks before the year-end holidays, is one of several retailer data thefts that have come to light in recent months.

Target in January said the hackers stole such personal information as addresses and telephone numbers of up to 70 million customers. Target said it now has found that at least 12 million shoppers had both their credit-card and some personal information stolen and the overlap is likely greater.

Write to Paul Ziobro at paul.ziobro@wsj.com

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