The unticketed passenger was ultimately removed from the plane, report

Authorities are investigating how a woman made it onto a Delta flight - and then had to be escorted off the plane - without a boarding pass or photo identification.

The woman boarded Atlanta-bound Flight 1516 at Orlando International Airport on Saturday morning. According to a police report, she sat in another passenger's seat. Jenni Clemons, a fellow passenger on the flight, told ABC News that she thought the woman had been accidentally assigned the wrong seat.

"She just said, 'I'm not moving,' " Clemons told ABC. "And she never broke her gaze from the back of the seat in front of her even when all the authorities were speaking to her."

When airline employees asked for the woman's boarding pass, she claimed she had thrown it away, the police report said.

The Transportation Security Administration confirmed that the woman went through screening at a checkpoint, but a spokeswoman would not answer questions about whether she showed identification or a boarding pass at that time.

The unticketed passenger was ultimately removed from the plane, according to the Orlando (Florida) Police Department report. Once she was off, officers asked her to show an ID and her boarding pass.

"I told her we could not help her if she could not produce her identification or a boarding pass," the report says. An officer told the woman she would need to leave the gate; at that point, they brought her out of the area and she got on an elevator to catch a shuttle, the report says.

Meanwhile, security officials had everyone get off the plane and go through a new security screening. By the time they all boarded again, the flight had been delayed for three hours, according to Delta.

"Delta apologizes to customers of Flight 1516 for the delay after a person not ticketed for that flight was removed from the aircraft," the airline said in a statement. "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. Safety and security is always our top priority."

The carrier would not discuss details of the investigation. In a statement, TSA also said it was working with law enforcement officials and would not release more information.

Jeff Price, an aviation professor at Metropolitan State University of Denver, said that despite the outcome, the woman did not pose much of a security risk because she had already been screened by TSA.

"To a certain extent, the system worked, because part of the vetting process is to figure out how many people boarded the plane, check it against the number of people that were supposed to board the plane and who they are, and make sure that those numbers add up before the plane takes off," said Price, who has written about aviation security. "It would have been a much greater risk, of course, had she not been screened."

He said he expects TSA to investigate its own screening and the airline to examine its process to see how the woman managed to board without a ticket. He said that the feat is "unfortunately not as difficult as it seems, because as soon as the ticket agent gets busy, sometimes they don't notice."

A Delta spokesman called the incident "incredibly rare."