A gunman opened fire on two people in a car outside an Oklahoma Walmart on Monday morning, killing them, before fatally shooting himself, authorities said. Police received a report of a shooting in the parking lot of a Walmart in Duncan at 9:53 a.m. Monday and responded a minute later, Duncan Police Chief Danny Ford said at a news conference.

Ford said it appeared the shooter opened fire with a handgun and at one point walked up to the car, but it wasn't clear whether he started firing at first from on foot or from another vehicle.

A man and a woman who were sitting in the driver's side and passenger seats were found fatally shot, Ford said. The gunman was found dead outside the car. Ford said the victims knew the suspect, but couldn't detail the relationship.

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The victims had been inside the Walmart and had just returned to their car when the shooting happened, Ford said. Authorities said the shooter had never entered the store. One of the victims was either a current or former Walmart employee, Ford said.

Bullet holes were visible in the windshield of the red, two-door car, the Associated Press reports. Ford said he wasn't sure how many shots were fired, but could "visibly" account for nine.

Law enforcement work the scene where two men and a woman were fatally shot Monday, November 18, 2019, outside a Walmart store in Duncan, Oklahoma. Sean Murphy / AP

Ford said police are looking for witnesses and collecting surveillance video. A motive remains unclear. "I have no idea what was going through his head," Ford said.

Authorities said no one in the store was threatened and it appeared to be an isolated incident. Walmart spokeswoman LeMia Jenkins says the shooting was "not an active shooter situation" and that the store was not evacuated.

Ford said investigators have tentative identifications of the dead but did not publicly release their names pending family notification.

"It's regrettable, obviously, there's not much you can say to families that have this kind of loss except we're sorry," Ford said.

Duncan Public Schools were briefly placed under lockdown. The school district told CBS News they were given the all-clear by police.

Duncan is about 80 miles southwest of Oklahoma City.