• Officials reportedly found bullet casings with the words "deny," "defend," and "depose" on them at Brian Thompson's murder scene.
• The words are similar to the title of a 2010 book about the insurance industry, "Delay Deny Defend."
• Police have not yet shared a motive for the shooting.

The gunman who shot Brian Thompson, the CEO of UnitedHealthcare, reportedly left behind a cryptic message at the crime scene.

Update: A "person of interest," 26-year-old Luigi Mangione, was arrested in connection with Thompson's death in Altoona, Pennsylvania, on Monday.

Multiple news outlets said that the shell cases found at the scene were inscribed with the words "deny," "defend," and "depose." These words are similar to the title of Jay M. Feinman's 2010 book "Delay Deny Defend: Why Insurance Companies Don't Pay Claims and What You Can Do About It," causing speculation that the shooter may have been referring to it.

The phrase "delay, deny, defend" is also common among lawyers who say that insurance companies delay the claims process with paperwork, deny claims that should be covered, and then defend themselves in court if a claimant pursues legal action.

The suspect of Thompson's shooting was still on the run as of Friday afternoon. Police haven't shared a motive behind the killing, which took place Wednesday morning.

Feinman, an author and professor emeritus at Rutgers, wrote about the insurance industry's evolution and shared advice for consumers on handling disputed claims in his book "Delay Deny Defend."

An NYPD spokesperson declined to comment when asked if police were investigating any link between the book and the shooting. The author also declined a request for comment.

Here are some of the key takeaways from Feinman's book.

There's only one mention of UnitedHealthcare by name.

While Feinman mentioned several top insurance companies by name throughout the book — State Farm and Allstate in particular — UnitedHealthcare only appeared once.

In the introduction, Feinman described how, in 2009, UnitedHealth, Aetna, Guardian, and other companies agreed to stop using certain databases to calculate fees for out-of-network treatment after being accused by then-New York Attorney General Andrew Cuomo of systematically lowballing patients.

Feinman says the insurance industry changed in the early 1990s.

In the intro to his book, Feinman wrote that insurance companies began to significantly reconsider the claims process in the 1990s when they "became a profit center rather than the place that kept the company's promise."

A major part of this shift occurred when insurance companies, including Allstate in 1992, hired consulting giant McKinsey & Company, he said.

McKinsey developed new strategies for handling claims and saw it as a "zero-sum game," Feinman writes. The insurance companies started using computer systems to estimate the amounts to be paid and deterring claimants from hiring lawyers, he said.

McKinsey declined Business Insider's request for comment.

Insurance companies aren't friends — but they're also not the enemy, he wrote.

Feinman said in the book that insurance companies aren't our friends — but they're not our enemies, either.

That's because companies must pay claims "pretty well most of the time" to stay in business.

"The point of view in this book is pro-consumer but it is not anti-insurance," Feinman writes. "Insurance is essential to our economic security." However, to serve as "the great protector of the standard of living of the American middle class, prompt and fair claim handling has to be the rule," he wrote.

'Understand your coverage. Understand the claims system. Get help if you need it.'

In Chapter 11, Feinman outlined what consumers can do to protect themselves while also seeking ways to cooperate with insurance companies.

He wrote that the responsibility to fix the system shouldn't fall to consumers alone. Legislators, regulators, and the courts must also step in, he wrote.

Feinman's advice boiled down to three key tenets: consumers should research the reputation of their agencies and policies carefully, understand the claims system, and seek legal recourse when necessary.