"Wanted" posters with the names and faces of health care executives have been popping up on the streets of New York. Hit lists with images of bullets are circulating online with warnings that industry leaders should be afraid.

The apparent targeted killing of UnitedHealthcare CEO Brian Thompson and the menacing threats that followed have sent a shudder through corporate America and the health care industry in particular, leading to increased security for executives and some workers.

In the week since the brazen shooting, health insurers have removed information about their top executives from company websites, canceled in-person meetings with shareholders and advised all employees to work from home temporarily.

An internal New York Police Department bulletin warned this week that the online vitriol that followed the shooting could signal an immediate "elevated threat."

Police fear that the Dec. 4 shooting could "inspire a variety of extremists and grievance-driven malicious actors to violence," according to the bulletin, which was obtained by The Associated Press.

"Wanted" posters pasted to parking meters and construction site fences in Manhattan included photos of health care executives and the words "Deny, defend, depose" -- similar to a phrase scrawled on bullets found near Thompson's body and echoing those used by insurance industry critics.

Thompson's wife, Paulette, told NBC News last week that he told her some people had been threatening him and suggested the threats may have involved issues with insurance coverage.

Investigators believe the shooting suspect, Luigi Mangione, may have been motivated by hostility toward health insurers. They are studying his writings about a previous back injury, and his disdain for corporate America and the U.S. health care system.

Mangione's lawyer has cautioned against prejudging the case. Mangione, 26, has remained jailed in Pennsylvania, where he was arrested Monday. Manhattan prosecutors are working to bring him to New York to face a murder charge.

UnitedHealthcare's parent company, UnitedHealth Group, said this week it was working with law enforcement to ensure a safe work environment and to reinforce security guidelines and building access policies, a spokesperson said.

The company has taken down photos, names and biographies for its top executives from its websites, a spokesperson said. Other organizations, including CVS, the parent company for insurance giant Aetna, have taken similar actions.

Government health insurance provider Centene Corp. has announced that its investor day will be held online, rather than in-person as originally planned. Medica, a Minnesota-based nonprofit health care firm, said last week it was temporarily closing its six offices for security reasons and would have its employees work from home.

Heightened security measures likely will make health care companies and their leaders more inaccessible to their policyholders, said former Cigna executive Wendell Potter.

"And understandably so, with this act of violence. There's no assurance that this won't happen again," said Potter, who's now an advocate for health care reform.

Private security firms and consultants have been in high demand, fielding calls almost immediately after the shooting from companies across a range of industries, including manufacturing and finance.

Companies have long faced security risks and grappled with how far to take precautions for high-profile executives. But these recent threats sparked by Thompson's killing should not be ignored, said Dave Komendat, a former security chief for Boeing who now heads his own risk-management company.

"The tone and tenor is different. The social reaction to this tragedy is different. And so I think that people need to take this seriously," Komendat said.

Just over a quarter of the companies in the Fortune 500 reported spending money to protect their CEOs and top executives. Of those, the median payment for personal security doubled over the last three years to just under $100,000.

Hours after the shooting, Komendat was on a call with dozens of chief security officers from big corporations, and there have been many similar meetings since, hosted by security groups or law enforcement agencies assessing the threats, he said.

"It just takes one person who is motivated by a poster -- who may have experienced something in their life through one of these companies that was harmful," Komendat said.

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Associated Press reporters Wyatte Grantham-Philips in New York and Barbara Ortutay in San Francisco, contributed to this report.

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The Associated Press Health and Science Department receives support from the Robert Wood Johnson Foundation. The AP is solely responsible for all content.