The Cobb County Police Department said the incident at the Six Flags Over Georgia park happened just before 16:00 local time (20:00 GMT) on Monday.
"Officers working at the park responded to an incident involving a male guest on a roller coaster," it said in a statement on Tuesday.
"Preliminary information gathered through witness and employee interview indicates Kariem was ejected from the ride and struck other patrons before falling beneath the attraction."
The ride involved has not been specified but aerial footage from news helicopters showed the Goliath rollercoaster stopped midway, and multiple witnesses have identified the ride as the one involved in the incident.
The office of the state's insurance and fire safety commissioner, which enforces amusement park safety rules in Georgia, is also conducting a review.
Officials have not held a news conference on the incident.
The Goliath ride opened in 2006, and it has an elevation of 200ft (60m), a 170ft plunge, a 540-degree spiral, and a speed of up to 70mph (112km/h), according to the park website. On its "thrill scale", Six Flags ranks Goliath at level four out of five.
The company said in a statement that it was "deeply saddened by the guest incident" and that "our thoughts and heartfelt condolences are with the guest's family, friends, and loved ones".
Nick Mendenhall said he witnessed the fatal moments in disbelief, watching the man come out of his seat and then fall into another cart on the ride before falling to the ground.
"It was just super, super bizarre, traumatic. I mean, it took me a couple of seconds, I feel like, to process what was going on," he told the AP. "I cant believe that I just saw that."
He said his mind instantly went to the trauma of all of those impacted by what he witnessed, including the man's loved ones and everyone who was on the ride with him.
One witness told NBC affiliate WXIA that the incident was captured in souvenir photos that were automatically taken of the rollercoaster while it was in motion. The BBC has not been able to verify this statement.
Connor Thompson told the BBC's US partner CBS News that he was in line for the ride when everyone started "yelling, clearly distressed".
"Everyone was kind of confused at first, and then there was someone who was yelling, 'Call 911! Someone fell out!" he said. "At first it was almost like I didn't believe it because I thought that was like impossible for something like that to happen."
He said he saw the cart come into the hub where others were waiting in line and could see the restraint was unlatched.
Another witness, CiCi Nelson, told CNN she, too, was in line to ride the Goliath when the rollercoaster returned and she saw an empty seat with a loose safety bar and a female covered in blood.
Nelson described a scene of hysteria, adding that one man was "freaking out, saying someone fell off".
Brittany Landry, who believed she saw Kariem ejected from the rollercoaster, told Fox News she was in tears.
"I broke because we were just on there," she said.
Kariem's family said he was a father, passionate educator and had survived two bouts of cancer.
His family told WXIA that he had survived cancer twice - the last stint in 2018 - and was in a car crash with his 10-year-old daughter three weeks before his death.
He lived in the nearby city of Atlanta and worked for Wesley International Academy, a public charter school, for more than 12 years as an IT manager, technology teacher and robotics coach.
"Mr. Kariem was deeply loved and respected, and his absence will be felt throughout our school community," the school said in a statement posted to its Facebook account.
"His love for Wesley, dedication to our students, and commitment to our school community were evident in everything he did."
Outside of work, Kariem enjoyed creating products with his laser engraver and 3D printers, as well as cooking, watching anime and spending time with family and friends, according to his profile on the school's website.
The death at Six Flags Over Georgia is at least the seventh there, according to the Doan Law Firm, which tracks amusement park accidents and represents victims.
According to the firm, this includes two people who had medical emergencies, two who were in restricted areas when they were struck by a ride, two who died in car-related accidents and one who died months after using the park's pool.
Six Flags operates 20 amusement parks, 14 water parks and nine resorts across Canada, the US and Mexico.
In the last 15 years, two deaths caused by falling from the park's rollercoasters led to lawsuits and settlements. One in 2011 involved an Iraq War veteran who was ejected from a ride in New York and hit his head on the car. In the other, in 2013, a woman slipped from her restraints on a rollercoaster in Texas and fell 75 feet.
In September, the Six Flags Magic Mountain park in California announced it was closing its X2 rollercoaster after more 100 people alleged that they had suffered brain injuries from the ride.
With additional reporting by Brandon Drenon