The words “deny,” “defend” and “depose” were discovered by investigators on bullet casings found at the crime scene where Brian Thompson, the CEO of major insurance company UnitedHealthcare, was shot, police sources told ABC News late Wednesday night .

According to police, Thompson was shot at close range in midtown Manhattan on Wednesday morning before he was scheduled to attend an investor conference.

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

NYPD investigators are working to determine whether the words were intended as a message from the shooter and a clue to his motive.

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

The moment was captured on video when the gunman walked up behind the CEO, pointed his gun at him and fired. A witness fled when Thompson stumbled and fell to the ground. The gunman then moved closer to Thompson and fired again before fleeing.

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

When the suspect was seen on video at the 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 was “shocked” by the “senseless murder.”

“Brian was an incredibly loving, generous and 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 we miss him dearly.”

Thompson, 50, was in New York City for UnitedHealthcare’s investor conference, which was scheduled to begin at 8 a.m., and his schedule was widely known, police sources said.

The conference was at the Hilton where he was shot, but he didn’t stay there, police said. The hotel is located at the intersection of West 54th Street and Sixth Avenue.

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

“People walked past the suspect, but he appeared to be waiting for his intended target,” Tisch said.

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

“The shooter then walks towards the victim and continues shooting. There appeared to be a weapon malfunction as he cleared the jam and started firing again,” he said.

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

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

Sources said investigators are working to determine whether the suspect had parked the bike somewhere nearby.

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

Investigators believe the shooter is not a professional killer, sources said.

The FBI, which has the most advanced technology for retrieving actionable data from cellphones, is helping in the investigation, the sources said.

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

Thompson left his hotel alone and appeared to have no security guards, police said.

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

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

“Brian was a valued colleague and friend to all who worked with him,” the company said. “We are working closely with the NYPD and ask for your patience and understanding during this difficult time. Our condolences go out to Brian’s family and everyone who was close to him.”

Thompson lived in Minnesota, where the company is based.

“This is terrible news and a terrible loss for the economy and health care in Minnesota,” Minnesota Gov. Tim Walz tweeted. “Minnesota sends 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.

ABC News’ Mark Crudele and Josh Margolin contributed to this report.