Delta and the Transportation Security Administration (TSA) were investigating how it happened, Fox 35 reported.
According to the passenger who didn't want to be identified, the woman refused to get up from the seat. "I also had a boarding pass for that seat, and she said very bluntly, 'I'm not moving.'"
The passenger told WFTV she asked for a flight attendant who came over and noted the woman wasn't listed on the flight's manifest.
The pilot and a supervisor then reportedly tried to remove the woman to no avail.
When they asked for her boarding pass, the woman said she threw it away and when they asked for her ID, she showed them a photo of herself on her phone, WFTV reported.
The passenger recalled a conversation between the woman and the flight attendant.
Well, I'm showing you a picture ID," the woman said. "Ma'am, that's not a government-issued ID. That's a photo," the flight attendant said. "Well, this is just as good," the woman said. "No, ma'am, it's not just as good," the flight attendant said.
"They eventually told her, 'You're breaking federal law,'" the passenger said, according to the outlet.
Soon after, the pilot called the police who were able to remove the woman. She reportedly left the plane cursing, to the confusion of many other passengers.
"We were stopped pretty quickly, and the pilot explained it was because the lady was not cooperating with TSA, and she did not have a ticket to any airline at all," the passenger told WFTV. "They could not figure out how she even got on the plane."
She said TSA agents and dogs searched the plane for about an hour, and workers patted down every passenger and searched their bags before the flight took off for Atlanta.
The TSA told the outlet the woman was screened but didn't provide more information. It's still unknown how the woman managed to get on the plane without a boarding pass or ID.
Delta released a statement following the incident.
"Delta apologizes to customers of flight 1516 for the delay after a person not ticketed for that flight was removed from the aircraft," Delta said. "Security officials then directed precautionary rescreen of everyone onboard. Delta is working with local law enforcement and the Transportation Security Administration on their investigation and we are conducting our own review of this as well."
Orlando Police Lt. Wanda Miglio, who was called to the airport just before 10:30 a.m. after getting reports of a "suspicious person," said the investigation was ongoing and the FBI has been notified of the incident, ABC News reported.