• UnitedHealthcare CEO Brian Thompson was fatally shot Wednesday in New York City.
• The words "deny," "defend," and "depose" were found on shell casings at the scene, reports said.
• Officials are investigating whether the words are related to a motive.

Police found the words "deny," "defend," and "depose" on casings from bullets used to kill the UnitedHealthcare executive Brian Thompson on Wednesday in New York City, reports said.

The words were found on casings at the scene in Manhattan, and police are investigating whether they indicate that the motive for the crime was the insurance company's response to a claim, ABC News first reported.

Thompson's wife, Paulette, told NBC on Wednesday that he had spoken to her about receiving death threats she believed were related to a "lack of coverage."

"There have been some threats," she told the network. "Basically, I don't know, a lack of coverage? I don't know details. I just know that he said there were some people that had been threatening him."

A spokesperson for UnitedHealthcare did not immediately respond to requests for comment on a potential link between the shooting and a coverage-denial issue. Paulette Thompson couldn't be reached by BI for comment.

The words on the cases are similar to the title of a 2010 book, "Delay Deny Defend," which is subtitled, "Why Insurance Companies Don't Pay Claims and What You Can Do About It."

The author of the book, Jay M. Feinman, a legal professor who specializes in insurance law, torts, and contract law, declined a request for comment.

Police are still searching for the shooter. Authorities have described the killing as a "targeted attack."

Thompson was gunned down at about 6:45 a.m. outside the Hilton hotel in midtown Manhattan. The shooter fled the scene before police arrived.

UnitedHealthcare is the largest private insurer in the US, and Thompson was in New York for an investor meeting when he was killed.

The "investor day" event was then canceled.