Even as UnitedHealth Group workers have been wrongly made to feel unsafe by a barrage of threats and vitriol over the past nine days, CEO Andrew Witty said in a New York Times op-ed Friday that insurers must do better in being transparent with patients when coverage for care is denied.

The company followed up Friday evening with a statement to correct what it called misinformation about the denial rate at its UnitedHealthcare business, the nation's largest health insurer.

About 90% of medical claims are paid upon submission, and of those requiring further review less than 1% are due to medical or clinical reasons, the company said.

"Health care is both intensely personal and very complicated, and the reasons behind coverage decisions are not well understood," Witty wrote in the guest column. "We share some of the responsibility for that. Together with employers, governments and others who pay for care, we need to improve how we explain what insurance covers and how decisions are made."

The comments follow a wave of public anger that crashed over the insurance industry online, especially on social media, since the killing of Brian Thompson, the UnitedHealthcare chief executive and Maple Grove resident shot repeatedly on a sidewalk in New York City.

The outrage was fueled by early reports on Thompson's death that quoted his wife as saying the executive had received threats, possibly over denials, and because the words "deny" and "delay" reportedly were found written on bullet casings recovered from the crime scene.

UnitedHealth Group confirmed Thursday night that shooting suspect Luigi Mangione did not have health insurance from UnitedHealthcare, contrary to speculation the 26-year-old might have been motivated by a coverage dispute with the company.

Safety concerns amid the animosity prompted two other health insurers in the Twin Cities to temporarily close offices this month.

Leaders of Minnetonka-based UnitedHealth Group have appreciated "an enormous outpouring of support" from some, Witty wrote, for Thompson as well as the company's UnitedHealthcare insurance division.

"Yet we also are struggling to make sense of this unconscionable act and the vitriol that has been directed at our colleagues who have been barraged by threats," he said. "No employees ... should have to fear for their and their loved one's safety."

In Minnesota, UnitedHealth Group employs about 19,000 people at its headquarters as well as the health insurance business and Optum, which runs clinics and manages pharmacy benefits.

In the op-ed, Witty echoed themes he started to send during an investor conference that was interrupted by word that Thompson had been killed on his way to the event. The company is well aware of problems with the U.S. health care system, he said, and is focused on driving change.

"We know the health system does not work as well as it should, and we understand people's frustrations with it," Witty wrote. "No one would design a system like the one we have. And no one did. It's a patchwork built over decades."

When making decisions on whether to deny coverage or pay for patient services, health insurers rely on clinical evidence to determine whether a treatment is safe and will bring the best patient outcome, Witty wrote.

"Highly inaccurate and grossly misleading information has been circulated about our company's treatment of insurance claims," the company said Friday. It also clarified that neither of Mangione's parents had their health insurance through UnitedHealthcare either.

For months before Thompson's killing, tensions over health insurance denials were on display in Minnesota during contract disputes between UnitedHealthcare and two large health systems in the state.

Bloomington-based HealthPartners and Duluth-based Essentia Health threatened to drop out of the insurance company's Medicare Advantage networks, saying its denial rate was excessive. The insurer called the allegations outlandish and untrue, while suggesting hospitals in contract disputes often try using patients as leverage to win higher reimbursement rates.

Ultimately, both health systems agreed to contracts so they'll stay in-network next year for UnitedHealthcare Medicare Advantage plans.

In October, a report by a U.S. Senate subcommittee found problems at three national health insurers including UnitedHealthcare, which reportedly denied prior authorization requests for post-acute care at three times the rate of other requests. The company said in response it still approves the vast majority of requests for post-acute care for patients after hospitalizations, and that it is federally required to give those claims heightened scrutiny.

In July, 11 people were arrested outside its Minnetonka headquarters during an event to spotlight coverage denials. The company responded by stressing the safety of its workers while asserting it had resolved the group's concerns for individual patients and was open to further dialogue.

Health policy experts say there's been a lack of comprehensive data on how frequently denials occur as well as the reasons for them. Surveys have shown public support for making more information available to patients.

In his editorial, Witty did not advance any specific proposals but called for greater understanding of the complexity of health care and the difficulty of change.

"While the health system is not perfect, every corner of it is filled with people who try to do their best for those they serve," Witty wrote.