The words “deny,” “defend” and “depose” were discovered by detectives on the shell casings found at the scene where Brian Thompson, the CEO of major insurance group UnitedHealthcare, was gunned down, police sources told ABC News late Wednesday evening.

Thompson was shot to death at point-blank range in Midtown Manhattan on Wednesday morning before he was set to attend an investor conference, according to police.

The masked gunman, who remains on the loose, carried out a “brazen, targeted attack” that was “premeditated,” NYPD Commissioner Jessica Tisch said at a news conference. But the motive remains unknown, police said.

NYPD detectives are working to determine whether the words were meant as a message from the shooter and a hint at his motive.

The shooter appeared to be lying in wait at the hotel where he shot Thompson around 6:40 a.m., police said.

Video captured the moment the gunman walked up behind the CEO, pointed his gun at him and fired. A witness fled as Thompson stumbled and fell to the ground. The gunman then walked closer to Thompson, firing more times before fleeing.

The shooter was seen on video at 5 a.m. the morning of the shooting outside Frederick Douglass Houses, a public housing project on the Upper West Side, sources told ABC News.

When the suspect was spotted on video at Frederick Douglass Houses, he was seen carrying what appeared to be an e-bike battery.

Thompson’s wife, Paulette Thompson, said in a statement that she’s “shattered” by the “senseless killing.”

“Brian was an incredibly loving, generous, talented man who truly lived life to the fullest and touched so many lives,” she said. “Most importantly, Brian was an incredibly loving father to our two sons and will be greatly missed.”

Thompson, 50, was in New York City for the UnitedHealthcare investors conference, which was scheduled to start at 8 a.m., and his schedule was widely known, police sources said.

The conference was being held at the Hilton outside of which he was shot, but he was not staying there, police said. The hotel is by the intersection of West 54th Street and Sixth Avenue.

The shooter arrived at the scene about five minutes before Thompson arrived, NYPD Chief of Detectives Joseph Kenny said at the news conference.

“People passed the suspect, but he appeared to wait for his intended target,” Tisch said.

After Thompson exited his hotel across the street and walked to the Hilton for the conference, the shooter stepped onto the sidewalk from behind a car and shot the CEO in the back, Kenny said.

“The shooter then walks toward the victim and continues to shoot. It appears that the gun malfunctions, as he clears the jam and begins to fire again,” he said.

The suspect fled on foot into an alley, where a phone believed to be linked to the shooter was recovered, police sources told ABC News.

He then fled on a bike and he was last seen riding into Central Park at 6:48 a.m., police said.

Detectives are working to determine whether the suspect had prepositioned the bike somewhere close by, according to sources.

Police believe a water bottle and candy wrapper left near the shooting scene are linked to the shooter and investigators are running tests to determine if there are any usable fingerprints or DNA, law enforcement sources told ABC News.

Detectives believe the gunman is not a professional killer, sources said.

The FBI, which has the most sophisticated technology for retrieving usable data from cellphones, is helping with the investigation, the sources said.

Thompson arrived in New York on Monday. Police are interviewing Thompson’s colleagues and family about any potential specific threats, Kenny said.

Thompson left his hotel by himself and didn’t appear to have a security detail, police said.

The victim’s hotel room has already been accessed by investigators, whose top priority is determining Thompson’s most recent conversations and movements, sources said.

UnitedHealthcare’s parent company, UnitedHealth Group, the largest health insurer in the world, said in a statement, “We are deeply saddened and shocked at the passing of our dear friend and colleague Brian Thompson.”

“Brian was a highly respected colleague and friend to all who worked with him,” the company said. “We are working closely with the New York Police Department and ask for your patience and understanding during this difficult time. Our hearts go out to Brian’s family and all who were close to him.”

Thompson lived in Minnesota, where the company is based.

“This is horrifying news and a terrible loss for the business and health care community in Minnesota,” Minnesota Gov. Tim Walz tweeted. “Minnesota is sending our prayers to Brian’s family and the UnitedHealthcare team.”

Police urge the public to call Crime Stoppers at 1-800-577-TIPS with any information.