(CNN) Investigators believe golf pro Gene Siller -- one of three men found fatally shot on an Atlanta-area golf course Saturday -- was killed "because he witnessed an active crime taking place," police said Tuesday.
Police didn't specify what that crime was, but have said that the two other men were found dead in the bed of a pickup truck on a green near where Siller was shot.
"It does not appear he (Siller) was targeted," but instead was killed because he "happened upon a crime in progress involving the unknown suspect and the two deceased males," the Cobb County Police Department said in a news release.
Investigators said they are following leads, and are still trying to determine why the shooting happened. They have not released the name of a suspect.
Cobb County police said they were called to a report of a person shot around 2:20 p.m. Saturday and found Siller -- the club's director of golf -- shot in the head near the green of the course's 10th hole.
A truck was on the green, and officers found the bodies of the two other men in the truck's bed -- both of whom also had been shot, police said.
Someone had driven to the green in the truck -- a white Ram 3500 pickup -- and shot Siller when Siller arrived to see what was going on, a member of the club told CNN affiliate WXIA. The shooter then fled, WXIA reported.
One of the slain men was Paul Pierson, the registered owner of the Ram 3500, police said. The name of the other slain man was not released.
Pierson and the other slain man "appear to have no relation to the location at all," Cobb County police said Tuesday.
Members of the country club are in the dark about why the killings happened, a friend of Siller's said Tuesday morning.
"This doesn't make any sense," Brian Katrek, a member of Pinetree Country Club in Kennesaw and an anchor of SiriusXM PGA Tour Radio, told CNN's "New Day" before Tuesday's news release from police.
"I think the reaction from members right now is still one of pretty profound shock," Katrek said.
Investigators told CNN they are asking residents to check their doorbell video for any clues.
The club will reopen Wednesday, though the 10th hole will be closed for play for now, a club official said in a letter to members Tuesday.
Club members and staff "are still grieving over the senseless loss of our friend Gene Siller," club official Lou Bottino's letter reads.
Siller's wife and other family members on Monday visited the 10th green "to see where this unimaginable tragic act occurred as part of their grieving," Bottino wrote.
Siller's wife added flowers to a growing memorial at the 10th green, the letter reads.
The Georgia State Golf Association expressed its condolences for Siller's death. "All of us at the GSGA are deeply saddened by the tragic events that occurred earlier today at Pinetree CC. Our thoughts and prayers are with Gene Siller's family and friends," the association tweeted.
Siller, 46, leaves behind a wife and two children, ages 6 and 7, according to the Pinetree Country Club.
"Tragedy has stricken the Georgia Section PGA in the loss of our Member, Gene Siller. Thoughts and prayers for his family and the Pinetree Country Club family," Georgia's Professional Golfers' Association tweeted.
PGA of America President Jim Richerson also issued a statement: "We are truly heartbroken to hear about the senseless murder that took place yesterday at Pine Tree Country Club in Georgia that took the life of PGA member Gene Siller," the statement said. "PGA of America sends our thoughts, prayers and deepest sympathy to his family, club and the Georgia PGA Community."
Sebastian Schutte, a friend of Siller's, said the community was in shock.
"It didn't hit me until later that this happened at our country club," he told CNN on Monday. "I still can't believe it."
He said Siller was well-liked and "treated everyone with respect."
Siller's family set up a GoFundMe page to help with finances.
Correction: Previous versions of this article had the wrong age for Gene Siller. He was 46.