Twice on Monday, false security alarms were triggered at Hartsfield-Jackson International Airport as a result of what officials are calling an “unknown mechanical issue.”

First, around 1 p.m., the alarm at the South Terminal security checkpoint led to a lockdown, halting the train system and shutting down three security gates, airport spokesman Reese McCranie said.

“The alarm activation was caused by an unknown mechanical issue,” McCranie said in a statement.

The shutdown was limited to about 20 minutes, but still caused between 10 and 25 Delta Air Lines flights to depart as many as 15 minutes behind schedule, Channel 2 Action News reported.

The second incident came sometime after 5 p.m., but did not cause as long a delay, McCranie said.

“As soon as there was the alarm, we deemed it a false alarm,” he said. “We have strong indications to believe it was a faulty switch. We’re still investigating.”

It’s unclear whether departures were impacted by the second incident.

McCranie said the shutdown of the airport trains and gates are part of the security plans should an alert be legitimate.

“Apart from the faulty switch, all the systems and protocols are functioning according to the way they should,” he said.

Monday wasn’t the first time this month that airport operations were stymied by emergency protocols.

In the early morning hours of Sept. 3, smoke from an escalator near the train level of the "T" concourse forced security lines to stretch through baggage claim, lasting up to 40 minutes at times.

Several Atlanta firefighters were slightly injured responding to the emergency, officials said.