CHICAGO — Hackers accessed personal information of 4.4 million Americans in a cyber incident affecting credit reporting agency TransUnion, the company revealed in regulatory filings.

The breach involved personal information stored in a third-party application, including names, Social Security numbers and dates of birth, according to notifications filed with attorneys general in Maine and Texas.

The incident began on July 28 and was discovered two days later, TransUnion told regulators. The company stressed that its core credit database and credit reports were not compromised. TransUnion said the breach did not affect its primary credit reporting systems.

The Chicago-based company is one of three major U.S. credit bureaus, along with Experian and Equifax. TransUnion has begun notifying a

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