The “pop, pop, pop” that some people mistook for gunfire at Daniel K. Inouye International Airport Tuesday was Plexiglas barriers and plastic posts being knocked to the floor by panicked passengers after a smoking carry-on bag was found at a security checkpoint, a Transportation Security Administration official said at a news conference Wednesday.

The scenario began about 2 p.m. when a passenger who had gone through TSA screening took his bag off the conveyor belt at checkpoint 3 in Terminal 2 (overseas terminal) and noticed his bag was smoking. He laid it on the floor, and “a couple of seconds later it started smoking significantly,” said Jenel Cline, TSA federal security director for the Pacific Region.

FBI investigators determined an external battery charger was the heating source that caused two laptops and a foam neck pillow in a carry-on bag to burn and smoke, Department of Transportation spokesman Tim Sakahara said. The DOT said Tuesday a laptop popped and started to smoke, apparently from a lithium battery.

The FBI is investigating the matter, but details may be difficult to determine since the battery is burned beyond recognition, he said.

Cline said TSA was running canine-enhanced screening in that lane, which did not require passengers to remove laptops from their carry-ons.

TSA released a 20-second video showing a sudden emission of a large cloud of smoke and two uniformed officers quickly leaving the scene at a TSA checkpoint. No passengers were in the area at the time.

Hundreds of panicked passengers bolted past nearby TSA checkpoint 4 into the sterile area, causing the entire Terminal 2 to be evacuated, passengers to be deplaned and then all passengers being required to go through TSA screening. The upheaval caused a delay of four hours, with Terminal 2 finally reopening at 6 p.m.

Cline said she reviewed video footage from the area. “There was no panic until after the checkpoint (3) was evacuated, sometime after passengers were leaving the area and about 20 seconds later,” she said. Passengers at the next checkpoint began running, and “that’s when we saw the panic.”

“Nobody knows who yelled, ‘Run! Hide! Fight! Active shooter!,’ we don’t know what actually was yelled or how that notification came about, what caused the panic. All I know is that there were some sounds — the pop, pop, pop sounds. Who actually called out anything is unknown at this time.”

When asked about passengers saying TSA agents were yelling those directions, Cline repeated she did not know the source.

Within five minutes, TSA determined through interviews that it was falling stanchions and not gunshots.

Cline said she imagines that while TSA was evacuating the area, someone pulled a bag and knocked down one stanchion, which are all connected.

“They make a crack when they fall,” she said, clapping her hands loudly.

She and Sakahara explained the reason for having to deplane passengers and to resecure the entire terminal was that it was unknown what caused the bag to smoke, whether it was a diversion, who the passenger was and whether it may have been part of a larger harmful plot.

Cline said checkpoint 4 TSA agents were allowing people through.

“Everyone in the airport did exactly what they were supposed to do yesterday,” Cline said. “In 20 seconds, it’s very difficult to react and say how we might do something differently.”

They agreed that better communication needs to be worked on as the hundreds of passengers who fled outside the terminal onto the roadway could not hear what was being said over the public address system inside the terminal. Cline said TSA and law enforcement officers were outside informing passengers that there was no active shooter and that they could go to the interisland terminal for TSA screening since it remained secure.

The state Sheriff Division and the FBI interviewed the passenger and investigated his smoking bag.

Cline said TSA conducts regular training in conjunction with the DOT for active shooter scenarios.

In April, an active shooter exercise was conducted with 40 law enforcement officers attending, as well as TSA, DOT, airlines and other stakeholders.

“While the active shooter was a false alarm, it still was a real-life scenario happening at the airport, affecting real-life planes and passengers,” Sakahara said in response to whether false alarm drills were conducted.

He added that the health, safety and security of passengers and the traveling community was a top priority for the DOT and all other agencies involved, and counted it as a successful operation.