TULSA, Oklahoma - The River Spirit Casino is back open Monday after a front window of the building was hit by a bullet Sunday night.

The bullet shattered a window pane near the casino's front entrance.

Investigators are still trying to find the person who fired that shot, but the general manager of the casino says he's been told there was only one bullet and it came from across the street.

The call came to police just after 8 p.m. Sunday—reports of gunshots fired at the River Spirit Casino.

Deborah Berryman was playing slots when she says chaos erupted.

"We heard these loud pops. it almost sounded like fireworks going off and people started screaming and running away from the front door as the security came forward to the front door," Berryman said.

General Manager Jerry Floyd said he was, "Actually, very pleased with our entire staff."

The bullet hit a window pane near the front door.

Floyd said the FBI, U.S. Marshals, and the Creek Nation Lighthorse Police are investigating.

He said they told him they believe the bullet was fired from the east side of Riverside Drive, near a wooded area and an apartment complex.

"The people that were reporting multiple sounds of multiple gunshots, it's theorized now that the initial sound that they heard was the projectile impacting the glass followed almost immediately by the tempered glass shattering," Floyd said.

Floyd said the casino has emergency plans for situations like this, which call for the casino to go on lock down and not let anybody in or out, then move the customers to an area as far away as possible from the scene. Some of the customers were also taken to a storm shelter, Sunday night.

Floyd said two customers had minor medical issues. One fell during the commotion and another had an anxiety attack.

"We do review all of our procedures and protocol on a regularly scheduled basis and, as I had mentioned previously, last night when the call went in they followed protocol to the letter," Floyd said.

Floyd said more than 800 people were in the casino when the shot was fired.

Investigators have not talked publicly about the case.