By Daniel Arkin, Becky Bratu and Alexander Smith, NBC News

A suspected gunman was found dead early Tuesday after at least six shots were fired at a New Jersey mall, police said.

The body of Richard Shoop was discovered in an under-construction area of the Garden State Plaza Mall in Paramus, N.J., at around 3:20 a.m. ET.

Authorities said a gunman had fired at least six rounds after entering the complex at around 9:20 p.m. ET on Monday. Thousands of shoppers fled for the building but no injuries were reported.

Citing police, NBC New York reported that Shoop was a 20-year-old from Teaneck, N.J.

Paramus Police Department Chief Kenneth Ehrenberg said Shoop had stolen the firearm, a modified gun which looked like an AK-47, from his brother.

Ehrenberg said early estimates suggested at least 400 people had been locked into stores in the 2.5 million square-foot shopping center. 

He told a press conference early Tuesday that officials were still doing a store-by-store search to get people out of the building. The operation could continue for the "rest of the night," Ehrenberg said.

Earlier, witnesses told how a man dressed in all black, wearing a helmet and armed with an assault rifle had opened fire inside the mall.

Clarice Forbes, who works at the Talbots store, told NBC New York that she spotted the gunman as he appeared to fire his rifle into the air.

"When he walked past our store, he looked directly in the store and strolled past," Forbes said. "It was very, very terrifying."

She added: "There were so many thoughts going through my head. I was so scared."

More than 500 law enforcement units from local, county and state agencies swarmed the scene after initial reports of multiple shots fired, police said.

Officials said the mall would be closed Tuesday. There are more than 200 stores and 50 restaurants inside the complex.

Paramus is a borough in Bergen County, the most populous county in New Jersey and roughly 20 miles southwest of Manhattan.