HARRIS COUNTY, Texas -

Shopping has resumed at a southwest Harris County Walmart, which was the scene of a HazMat scare late Sunday night. Now, police are searching for the two people they say walked in and released something into the air.

There were more than 150 people inside the Walmart store off of Highway 6 near Westpark Drive when all of the sudden many of them of them started coughing. Others also had a hard time breathing.

"I started coughing and I saw a lot of people covering their noses and coughing," said Farah Mohamoud. "I looked at my kids, and they started to get tears in their eyes. Then the [store employees] started asking us to go outside, go, go, go."

Harris County sheriff's deputies responded to the store with HazMat crews and cleared the area. They determined the air quality had returned to acceptable levels but said the two suspects who caused the scare had left the area.

"At this time, all we know is that two unknown suspects came into the store and released some type of chemical that caused a reaction to some of the customers," Harris County Sgt. B. Johnson explained. “Some became ill with headaches and nausea.”

Police said three people were transported to West Houston Hospital because of breathing problems. All of those treated overnight are expected to make a full recovery.

A few hours after the evacuation, crews determined the air quality was safe and the store was reopened.

Investigators say they don't know if the men sprayed the unknown substance as a prank or to cover up a crime such as shoplifting.

Detectives said there are security cameras inside and outside of the store; those images are being reviewed as they search for the two suspects responsible.

Authorities ask that anyone who knows the identity or location of the suspects in the case to contact the sheriff's office via iWatch Harris County.