A store-to-store search was under way early Tuesday for a gunman who opened fire as a busy northern New Jersey mall was about to close, authorities said.

Authorities said only one shell casing was found at the Westfield Garden State Plaza Mall in Paramus, but witnesses said they heard as many as a half-dozen shots fired about 9:30 p.m. Monday.

Panicked shoppers raced from the mall as SWAT teams and police agencies from across New Jersey converged there, locking down the mall and trapping thousands of people inside as authorities searched for the gunman. There were no immediate reports of injuries.

"I just saw a guy walking very calmly in a black suit, with some kind of backpack, looked like he had two gas balloons on him in the back and he was carrying a rifle, with a helmet," a witness told WABC-TV. "He was walking very calming, just shooting at people running all around . . . It was very bizarre. I think he was shooting up at the cameras of the mall."

Shortly after 11 p.m., authorities said even though they continued to search, they believed the shooter was no longer in the mall and that only one shell casing had been found.

"All we can confirm right now is we found one shell casing, which we believe is from a long-arm rifle," Office of Emergency Management deputy coordinator James Tedesco told reporters. He said authorities were going through surveillance video, but added that it was not clear if the gunman had targeted security cameras.

Tedesco said police were looking for other shell casings to try to determine where the shooter went.

"Everyone is safe," Paramus Mayor Richard LaBarbiera told reporters. "No one has been injured. Right now we have police agencies from all over the state who are now going through the mall, sweeping, to ensure whatever people are in the mall at this time . . . are evacuated in a safe and orderly way."

Workers and shoppers were kept inside stores until rescuers could reach them and escort them to safety, officials said.

"Multiple gunshots were heard. We believe it is one shooter. I know there is an active shooter near the Nordstrom area. The mall is being evacuated," Bergen County Chief of Staff Jeanne Baratta told Reuters.

One witness told CNN she was in a parking lot by the Nordstrom department store and she heard the shooting was inside the store. "There were three shots and all of a sudden people are coming out and the cops were pushing people back," she said.

A store employee, Eddie Kahmann, told CNN he was told to stay in his store in the mall as police searched. He said he had heard "six or seven shots."

Mall officials could not be reached early Tuesday.

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