Atlanta ? Officials shut down all security checkpoints at the nation’s busiest airport for about two hours after a “suspicious image” was detected in a screening machine.

After a hand search of bags, nothing matching the image was found, said Willie Williams, Transportation Security Administration director at Hartsfield-Jackson International Airport. He declined to say what the image appeared to be but said authorities continued to analyze it.

Earlier in the day, TSA spokeswoman Amy von Walter said a possible homemade bomb had been detected.

A screener notified a supervisor of the suspicious image at 1:15 p.m., Williams said. The airport closed security checkpoints and grounded departing flights.

Checkpoints reopened at 3:40 p.m., while airport officials still “couldn’t say for certain we had a threat,” Williams said.