A man shot and killed his estranged wife and her new boyfriend in a Walmart parking lot in Oklahoma before turning the gun on himself when an armed citizen intervened, police said.

According to the Oklahoma Highway Patrol, the man was wearing all black when he opened fire outside the supercenter in Duncan, which is located at 3393 North 81 Highway. Duncan is about 80 miles south of Oklahoma City.

Officers from the Duncan Police Department said the wife and her boyfriend were found dead inside their car.

Family members at the scene say the shooting was sparked by a domestic dispute between the woman and her husband, but didn't go into further details.

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A man shot and killed his estranged wife and her new boyfriend in a Walmart parking lot (pictured) in Oklahoma before turning the gun on himself when an armed citizen intervened

A witness claimed that the shooter fired nine rounds, killing the two victims. Aerial footage (pictured) shows police cordoning off a large area of the parking lot

Officers from the Duncan Police Department said the wife and her boyfriend were found dead inside a red car. Police are seen looking at a red vehicle (pictured, with the door open, left) that appears to have multiple bullet holes in the windshield. A body is seen lying next to the car

According to local KOCO News, a witness claimed that the shooter fired nine rounds into the car, killing the two victims.

When the gunman was confronted by an armed citizen, he then turned the gun on himself.

Shots rang out at the Walmart just before 10am Monday morning, and authorities responded to the scene a short time later.

Aerial footage from Reuters shows police cordoning off a large area of the parking lot.

Police are also seen looking at a red vehicle that appears to have multiple bullet holes in the windshield. A handgun was found at the scene.

No staff members were involved in the shooting and the store was not evacuated, Walmart spokeswoman LeMia Jenkins said in an email.

'This was an isolated incident in the parking lot and was not an active shooter situation,' Jenkins said.

A family member reacts at the scene of the fatal shooting in the parking lot of a Walmart in Duncan, Oklahoma, on Monday

Family members (left) arrived to the scene of the fatal shooting on Monday as investigators continued to secure evidence (right)

Walmart representatives said in the statement that they are referring all additional questions to law enforcement because it's still an 'active police investigation'.

A body is seen lying next to the vehicle at the crime scene under the sheet.

During the shooting, administrators for the Duncan Public School placed local schools on lockdown and alerted parents.

'Duncan Public Schools is aware of the report of a shooting at Duncan Wal-Mart,' officials initially wrote.

'As always we are taking every precaution to protect our staff and students. At this time all schools are in lockdown due to this report. Schools will operate as normal, but visitors will not be admitted until police report it is safe.'

The lockdown was lifted just before 11.30am.

State Representative Forrest Bennet tweeted about the shooting: 'The closer it is, the more it hurts. Duncan once again suffers from gun violence.

'I’ll be thinking of the victims & their families and, as we learn more about the motivation of the shooter, looking for ways to prevent this from happening again.'

Monday's shooting comes just hours after four people were shot dead in Fresno, California.

The car's windshield had a total of nine bullet holes, six on the passenger side and three on the driver's side

A handgun lays on the ground and is marked as evidence at the scene of the shooting

The body of the suspected gunman lays on the ground after the shooting in a Walmart parking lot on Monday

According to the Oklahoma Highway Patrol, the husband was wearing all black when he opened fire on the couple outside the supercenter in Duncan (depicted above), which is located at 3393 North 81 Highway

Officers collecting evidence and interviewing witnesses at the scene of the crime on Monday

Police are searching for suspects and trying to determine a motive for the shooting deaths of four men who were at a backyard party.

Six others were wounded during the shooting as they watched football on TV.

Fresno Police Lt Bill Dooley said 'unknown suspects' sneaked into the backyard at about 6pm Sunday and fired into the crowd.

Fresno Deputy Police Chief Michael Reid said 10 people were shot. Three were found dead in the backyard and a fourth person died at the hospital. The six others are expected to survive and are recovering at a hospital.

The victims were men ranging in age from 25 to 35, Reid said.

The Oklahoma shooting also comes just months after a gunman opened fire inside a Walmart in El, Paso, Texas, killing 22 people.

In the wake of that shooting which occurred in August, along with an Ohio shooting that happened within 24 hours of one another, Walmart announced that ammunition for assault rifles and handguns would no longer be sold in its 4,177 stores.

At the time, the company also announced that customers would be banned from openly carrying firearms inside its stores.

In the case of the recent shooting, Oklahoma has long been an 'open' or 'concealed' carry state, allowing most adults to carry guns openly without a permit as long as they have a 'legitimate purpose and aren't violating other laws'.

As of November 1, 2019, Oklahoma became even more gun friendly by passing a law to eliminate the license requirement for carrying concealed handguns.