Medical records, driver's license and Social Security numbers, and other personal information of Texas Health Resources patients may have been accessed unlawfully, the Arlington-based health system is warning.

Texas Health is one of North Texas' largest provider groups with more than 350 community access points. Over 1.7 million patients visit its physician offices annually, public documents show.

In a notice posted Friday, Texas Health said an unauthorized third party may have gained access to email accounts last October. As a result, the system says, it has begun notifying individual patients in writing. Texas Health told The Dallas Morning News that fewer than 4,000 people are affected.

The breach in patient data is part of a larger incident affecting multiple U.S. entities, said the notice shared with consumers. It did not go into many specifics. Texas Health says law enforcement identified the potential breach in January but asked that patients not be alerted while an investigation was underway.

"We deeply regret any inconvenience or concern this may cause our patients," said the posting on the website of the Texas Health Physicians Group, which employs approximately 900 physicians, physician assistants, and nurse practitioners.

“Texas Health is continuously working to implement safeguards and enhance information security monitoring,” the system said about efforts to prevent breaches in the future.

Access to medical data has been described as a new frontier for hackers, especially as the health care industry shifts from paper files to electronic health records.

The records of more than 4 million Texans have been exposed since 2009 when the U.S. Department of Health and Human Services began to keep track of incidents that left the health data of 500 or more patients at risk at once.

About 20 incidents affecting over 400,000 patients statewide are part of active investigations by the federal officials, the data show.

Health care data can be valuable on the dark web. It can be used to help an unhealthy person fake a clean bill of health or assist a person trying to get prescription drugs, among other things.

The incident with Texas Health is not currently listed in the federal database.

Texas Health said there is no indication that any information has been misused but says it began to mail letters to affected patients on Friday out of “an abundance of caution.”

The system will offer one year of free credit monitoring to those whose Social Security numbers were breached. People who feel they have been affected can call (855) 331-3705 on Monday through Friday between 8 a.m. and 8 p.m.

“We recommend affected patients review any statements they receive from their health insurer. If patients see charges for services they did not receive, please contact the insurer immediately,” Texas Health said.