UPDATED | December 22, 2017  11:11 AM

DALLAS (CBSDFW.COM) – Smoke forced the evacuation of Dallas Love Field Airport this morning, but things were back to normal by lunchtime.

The incident happened around 4:30 a.m. as passengers began arriving for the busy holiday travel weekend. Officials with the City of Dallas say it was smoke from an over-heated A/C unit that caused the evacuation.

Officials with Dallas Fire Rescue confirm they received a report of smoke in the terminal. Traveler Tim Ebbeck said, “The [fire] alarm went off and then they told us to go outside.”

Passengers were forced out of the building while fire crews made their way in. Video and pictures from the scene showed fire engines arriving as crowds waited outside.

Once the cause of the smoke was determined and dealt with the airport was cleared and returned to normal operations. Passengers were allowed back in after about 40 minutes, but delays were long because TSA workers had to re-screen all passengers.

One woman was glad to get back inside and thankful the airport promised to assist everyone. “It was pretty stressful, but they say they’re going to get everyone on their flights and not leave without anyone,” she said.

Southwest Airlines, the biggest carrier at the airport, issued a statement that said, in part –

“We appreciate our passengers’ patience as we work through this and work to get them to their destinations safely and as quick as possible.”

For hours after the evacuation the airport was advising passengers to allow at least 90 minutes to get through the security checkpoint.

Love Field spokes person Anastasia Reed said, “We’re doing everything on our end to make sure that this is a quick process to get everyone back in. But we’re asking everyone to please be patient with us. We apologize for the inconvenience, but it’s going to take some time for us to get back to normal.”