NEW YORK (AP) — Police don't know who he is, where he is, or why he did it.

As the frustrating search for UnitedHealthcare CEO Brian Thompson’s killer got underway for a fifth day Sunday, investigators reckoned with a tantalizing contradiction: They have troves of evidence, but the shooter remains an enigma.

One conclusion they are confident of, however: It was a targeted attack, not a random one.

They know he ambushed Thompson at 6:44 a.m. Wednesday as the executive arrived at the Hilton for his company’s annual investor conference, using a 9 mm pistol that resembled the guns farmers use to put down animals without causing a loud noise. They know ammunition found near Thompson’s body bore the words “delay,” “deny” and “depose,” mimicking a phrase used by insurance industry critics.

The fact that the shooter knew UnitedHealthcare group was holding a conference at the hotel and what route Thompson might take to get there suggested that he could possibly be a disgruntled employee or client, NYPD Chief of Detectives Joseph Kenny said.

Over the weekend, police divers were seen searching a pond in Central Park, where the killer fled after the shooting. Officers have been scouring the park for days for any possible clues and found his backpack there Friday. They didn’t immediately reveal what, if anything, it contained but said it would be tested and analyzed.

Early Sunday afternoon, police declined to comment on the contents of the backpack, or on the results of the search in the pond, saying no updates were planned.

The bag’s apparent manufacturer did not immediately respond to questions from The Associated Press.

Investigators have urged patience, saying the process of logging evidence that stands up in court isn’t as quick as it looks like on TV.

Hundreds of detectives are combing through video recordings and social media, vetting tips from the public and interviewing people who might have information, including Thompson’s family and coworkers and the shooter’s randomly assigned roommates at the Manhattan hostel where he stayed.

Investigators caught a break when they came across security camera images of an unguarded moment at the hostel in which he briefly showed his face.

Retracing the gunman’s steps using surveillance video, police say, it appears he left the city by bus soon after the shooting outside the New York Hilton Midtown. He was seen on video at an uptown bus station about 45 minutes later, Kenny said.

With the high-profile search expanding across state lines, the FBI announced late Friday that it was offering a $50,000 reward for information leading to an arrest and conviction, adding to a reward of up to $10,000 that the NYPD has offered. Police say they believe the suspect acted alone.

Police distributed the images to news outlets and on social media but so far haven’t been able to ID him using facial recognition — possibly because of the angle of the images or limitations on how the NYPD is allowed to use that technology, Kenny said.

Late Saturday, police released two additional photos of the suspected shooter that appeared to be from a camera mounted inside a taxi. The first shows him outside the vehicle and the second shows him looking through the partition between the back seat and the front of the cab. In both, his face is partially obscured by a blue, medical-style mask.