Target Corp., scheduled to testify tomorrow about a data breach affecting millions of customers, plans to tell lawmakers it had clues about the attack weeks before responding and is exploring why it took so long to react.

After intruders entered Target's systems on Nov. 12, some of their activities were detected and evaluated by security professionals, according to prepared remarks from Chief Financial Officer John Mulligan that were reviewed by Bloomberg. That was a month before the company was alerted to suspicious activity by the U.S. Justice Department, leading to an internal investigation that confirmed a breach on Dec. 15.

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