CHICAGO (CBS) — A man turned himself in to police overnight after a 45-year-old man was shot and killed while working out in a gym in a luxury high-rise in the Fulton River District neighborhood.

UPDATED STORY: Wheelchair-Bound Man Charged With Killing Neighbor Working Out At Luxury Gym

Witnesses said a man in a wheelchair went into the gym on the eighth floor of at an apartment building in the 500 block of West Kinzie Street around 7 p.m., and shot Darrin Joss in the head as he was working out on a treadmill.

Around 3:30 a.m. Thursday, a man in a wheelchair turned himself in at Chicago Police Headquarters. A Dodge Magnum matching the description of the vehicle witnesses said the gunman was driving also was seen parked outside.

Witnesses said the suspect rolled into the gym shortly after 7 p.m., and fired two shots at Joss as he was on a treadmill.

“I had my headphones in, so … I had music blasting in my ears, but I hear, I felt the ground just shake, and then I thought, I literally thought something just exploded in the gym, or someone dropped a weight, and then when it happened the second time, I was like, ‘Something’s not right,’ and then everyone was getting down on the floor. So I’m just on the floor, and I look to my left, and then this guy is laying down, just covered in blood,” Alexa Regimand said.

Regimand said she was “in complete shock.”

“I’ve never ever heard gunshots. I’ve never been around a gun. I thought a yoga ball exploded, and it happened twice. It happened twice, and on the second shot is when everyone, like, went down, and I just looked and saw the guy on the machine behind me was just covered in blood. And so we all just took off running,” she said.

About a dozen people ran for their lives after the gunman opened fire.

“I was actually supposed to be in there training a client. Sorry, I’m getting a little worked up. You just don’t think this would happen. I knew both of the guys. The man who actually did it is super-friendly,” Loren Belettini said.

Joss was taken to Northwestern Memorial Hospital after the shooting, and was pronounced dead at 7:48 p.m., according to the Cook County Medical Examiner’s office. His friends said he was a former football player and a personal trainer.

Patrick Durr, who lives in the building, said he is aware that there have been issues between the man in the wheelchair and several other residents of the building, but said he personally has found the man to be “pleasant” and said many residents routinely held doors for him.

Witnesses said the gunman was involved in a heated argument with Joss earlier in the evening.

“I heard that it was a dispute about a girlfriend,” Matt Budreau said.

“I overheard a man kind of getting mad at another man for flirting with his girlfriend,” Gina Genero said.

Akeem Hunter said he was playing basketball with the man in the wheelchair just hours before the shooting.

“I was talking to the guy an hour before the shooting. I don’t know what happened prior to the shooting, or whatever made him upset, but you just never know with people nowadays,” he said.

Police were reviewing surveillance video from inside the building and the parking garage.

No charges had been filed as of late Thursday morning.