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

“Prior authorisation” does not seem like a phrase that would generate much passion.

But on a hot day this past July, more than 100 people gathered outside the Minnesota headquarters of UnitedHealthcare to protest against the insurance firm’s policies and denial of patient claims.

“Prior authorisation” allows companies to review suggested treatments before agreeing to pay for them.

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

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

Unai Montes-Irueste, media strategy director of the Chicago-based advocacy group, said those protesting had personal experience with denied claims and other problems with the healthcare system.

“They are denied care, then they have to go through an appeals process that’s incredibly difficult to win,” he told the BBC.

The latent anger felt by many Americans at the healthcare system – a dizzying array of providers, for profit and not-for-profit companies, insurance giants, and government programmes – burst into the open following the apparent targeted killing of Thompson in New York City on Wednesday.

Thompson was the CEO of UnitedHealthcare, the insurance unit of health services provider UnitedHealth Group. The company is the largest insurer in the US.

Police are still on the hunt for the suspected killer, whose motivation is unknown, but authorities have revealed messages written on shell casings found at the scene.

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

A scroll through Thompson’s LinkedIn history reveals that many were angry about denied claims.

One woman responded to a post the executive had made boasting of his firm’s work on making drugs more affordable.

“I have stage 4 metastatic lung cancer,” she wrote. “We’ve just left [UnitedHealthcare] because of all the denials for my meds. Every month there is a different reason for the denial.”

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

“There had been some threats,” Paulette Thompson said. “Basically, I don’t know, a lack of [medical] coverage? I don’t know details.”

“I just know that he said there were some people that had been threatening him.”

A security expert says that frustration at high costs across a range of industries inevitably results in threats against corporate leaders.

Philip Klein, who runs the Texas-based Klein Investigations, which protected Thompson when he gave a speech in the early 2000s, says that he’s astonished the executive didn’t have security for his trip to New York City.

“There’s lot of anger in the United States of America right now,” Mr Klein said.

“Companies need to wake up and realise that their executives could be hunted down anywhere.”

Mr Klein says he’s been inundated with calls since Thompson was killed. Top US firms typically spend millions of dollars on personal security for high-level executives.

In the wake of the shooting, a number of politicians and industry officials expressed shock and sympathy.

Michael Tuffin, president of insurance industry organistion 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, health care professionals, associations, government officials and other caring people”.

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

Those reactions ranged from acerbic jokes (one common quip was “thoughts and prior authorisations”, a play on the phrase “thoughts and prayers”) to commentary on the number of insurance claims rejected by UnitedHealthcare and other firms.

At the extreme end, critics of the industry pointedly said they had no pity for Thompson. Some even celebrated his death.

The online anger seemed to bridge the political divide.

Animosity was expressed from avowed socialists to right-wing activists suspicious of the so-called “deep state” and corporate power. It also came from ordinary people sharing stories about insurance firms denying their claims for medical treatments.

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

He said his group campaigned in a “nonviolent, democratic” way – but he added he understood the bitterness online.

“We have a balkanised and broken healthcare system, which is why there are very strong feelings being expressed right now by folks who are experiencing that broken system in various different ways,” he said.

Mr Tuffin, head of the health insurance trade association, condemned any threats made against his colleagues, describing them as “mission-driven professionals working to make coverage and care as affordable as possible”.

The posts underlined the deep frustration many Americans feel towards health insurers and the system in general.

“The system is incredibly complicated,” said Sara Collins, a senior scholar at The Commonwealth Fund, a healthcare research foundation.

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

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

Not only is the US health system complicated, it’s expensive, and huge costs can often fall directly on individuals.

Prices are negotiated between providers and insurers, Ms Collins says, meaning that what’s charged to patients or insurance companies often bears little resemblance to the actual costs of providing medical services.

“We find high rates of people saying that their healthcare costs are unaffordable, across all insurance types, 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 truly have a medical debt crisis.”

A survey by researchers at health policy foundation KFF found that around two-thirds of Americans said insurance companies deserve “a lot” of blame for high healthcare costs. Most insured adults, 81%, still rated their health insurance as “excellent” or “good”.

Christine Eibner, a senior economist at the nonprofit think tank the RAND Corporation, said that in recent years insurers have been increasingly issuing denials for treatment coverage and making use of prior authorisations to decline coverage.

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

“On top of that, people face out-of-pocket costs, which could easily be in the thousands of dollars,” she said.

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

Last year, UnitedHealthcare settled a lawsuit brought by a chronically ill college student whose story was covered by news site ProPublica, which says he was saddled with $800,000 of medical bills when his doctor-prescribed drugs were denied.

The company is currently fighting a class-action lawsuit that claims it uses artificial intelligence to end treatments early.

The BBC has contacted UnitedHealth Group for comment.

With reporting by Tom Bateman