The “brazen and targeted” killing of health insurance executive Brian Thompson, CEO of UnitedHealthcare, outside a New York hotel this week shocked America. The response to the crime also revealed simmering anger against a trillion-dollar industry.

“Prior authorization” doesn’t seem like a phrase that would stir much passion.

But on a hot day last July, more than 100 people gathered outside UnitedHealthcare’s headquarters in Minnesota to protest the insurance company’s policies and denials of patient claims.

“Prior authorization” allows companies to review proposed treatments before agreeing to pay for them.

Eleven people were arrested for blocking a road during the protest.

Police records show they came to the rally organized by the People’s Action Institute from across the country, including Maine, New York, Texas and West Virginia.

Unai Montes-Irueste, media strategy director for the Chicago-based advocacy group, said protesters had personal experiences with denied claims and other problems with the health care system.

“They are denied care, then have to go through an appeal process which is incredibly difficult to win,” he told the BBC.

Many Americans’ latent anger at the health care system — a dizzying array of providers, for-profit and nonprofit, insurance giants and government programs — came to the surface after the apparently targeted killing of Thompson in New York City on Wednesday.

Thompson was CEO of UnitedHealthcare, the insurance unit of healthcare provider UnitedHealth Group. The company is the largest insurer in the USA.

Police are still searching for the suspected killer, whose motive is unknown, but authorities have revealed messages written on bullet casings found at the crime scene.

The words “deny,” “defend” and “depose” were discovered on the casings, which investigators believe could refer to tactics that critics say insurance companies use to avoid payouts and increase profits.

A look at Thompson’s LinkedIn history shows that many were upset about the denied claims.

A woman responded to a post in which the manager praised his company’s work to make medications more affordable.

“I have stage 4 metastatic lung cancer,” she wrote. “We just left (UnitedHealthcare) because my medications kept getting rejected. Every month there is a different reason for rejection.”

Thompson’s wife told NBC that he had received threatening messages before.

“There were some threats,” Paulette Thompson said. “Basically I don’t know, a lack of (medical) coverage? I don’t know the details.”

“All I know is that he said there were some people who threatened him.”

A security expert says frustration with high costs across a range of industries inevitably leads to threats for business leaders.

Philip Klein, head of Texas-based Klein Investigations, which protected Thompson during his speech in the early 2000s, says he was amazed that the executive had no security when he traveled to New York City.

“There is a lot of anger in the United States of America right now,” Klein said.

“Companies need to wake up and realize that their leaders can be hunted anywhere.”

Mr. Klein says he has been inundated with calls since Thompson’s killing. Leading U.S. companies typically spend millions of dollars on the personal security of senior executives.

After the shooting, numerous politicians and industry representatives expressed shock and sympathy.

Michael Tuffin, president of insurance industry organization Ahip, said he was “heartbroken and horrified by the loss of my friend Brian Thompson.”

“He was a devoted father, a good friend to many, and a refreshingly candid colleague and leader.”

In a statement, UnitedHealth Group said it had received many messages of support from “patients, consumers, healthcare professionals, associations, government officials and other caring people.”

But online, many people, including UnitedHealthcare customers and users of other insurance services, reacted differently.

These reactions ranged from caustic jokes (a common joke was “thoughts and prior authorizations,” a play on the phrase “thoughts and prayers”) to comments about the number of insurance claims denied by UnitedHealthcare and other companies.

At the very end, industry critics explicitly said they had no pity for Thompson. Some even celebrated his death.

The online anger seemed to bridge the political divide.

Avowed socialists expressed hostility toward right-wing activists who were suspicious of the so-called “deep state” and corporate power. It also came from ordinary people telling stories about insurance companies denying their claims for medical treatments.

Mr Montes-Irueste of People’s Action said he was shocked by the news of the murder.

He said his group had conducted the campaign “non-violently and democratically” – but he added he understood the bitterness online.

“We have a balkanized and broken health care system, which is why people who are experiencing this broken system in different ways are expressing very strong feelings right now,” he said.

Mr. Tuffin, chairman of the Health Insurance Association, condemned any threats against his colleagues, describing them as “mission-oriented professionals working to make insurance coverage and care as affordable as possible.”

The posts underscored the deep frustration many Americans feel toward health insurers and the system in general.

“The system is incredibly complicated,” said Sara Collins, senior scientist at the Commonwealth Fund, a health research foundation.

“Just navigating and understanding how to stay covered can be challenging for people,” she said. “And everything seems fine until you get sick and need your plan.”

Recent Commonwealth Fund research found that 45% of insured working-age adults were billed for something they thought should have been free or covered by insurance, and fewer than half of those reported suspected billing errors , have challenged this. And 17% of respondents said their insurer denied coverage for treatment recommended by their doctor.

The U.S. health care system is not only complicated, it is expensive, with enormous costs often falling directly on the individual.

Prices are negotiated between providers and insurers, Ms. Collins says, meaning the costs billed to patients or insurance companies often bear little resemblance to the actual cost of providing medical services.

“We find that many people say their health care costs are unaffordable, across all types of insurance, even (government-funded) Medicaid and Medicare,” she said.

“People accumulate medical debt because they can’t pay their bills. This is unique to the United States. We really have a medical debt crisis.”

A survey by researchers at health foundation KFF found that about two-thirds of Americans said insurance companies bear “a lot” of blame for high health care costs. Most insured adults, 81%, continued to rate their health insurance as “very good” or “good.”

Christine Eibner, senior economist at the nonprofit think tank RAND Corporation, said insurers have increasingly rejected treatment reimbursements in recent years and used prior authorizations to deny coverage.

She said the premiums were about $25,000 (£19,600) per family.

“In addition, people face out-of-pocket costs that can easily run into thousands of dollars,” she said.

UnitedHealthcare and other insurance providers have faced lawsuits, media scrutiny and government investigations over their practices.

Last year, UnitedHealthcare settled a lawsuit brought by a chronically ill college student whose story was reported by the news site ProPublica. It says he was saddled with $800,000 in medical bills because he was denied medication prescribed by his doctor.

The company is currently battling a class action lawsuit claims to use artificial intelligence to end treatments early.

The BBC has contacted UnitedHealth Group for comment.

With reporting by Tom Bateman