A stowaway evaded multiple airport security checkpoints and flew aboard a Delta Airlines flight from New York to Paris Tuesday evening, authorities said – a shocking breach that raised serious alarm over airport security.

The individual, who did not have a boarding pass, completed security screening and bypassed two identity verification and boarding status stations to board the aircraft, according to a spokesperson for the US Transportation Security Administration (TSA). The person was not carrying any prohibited items.

It’s unclear how the person, a U.S. green card holder, bypassed the boarding stations. Delta declined to provide additional details as the investigation continued.

Passengers weren’t alerted to the stowaway on Delta flight 264 from New York’s JFK International Airport to Paris Charles de Gaulle Airport until the plane landed in Paris, said Rob Jackson, who was a passenger on the flight.

“Apparently, she hid in a lavatory all the way at the rear of the aircraft when we departed JFK,” Jackson told CNN.

“The first announcement to passengers that there was a problem was when we parked at the gate and they instructed us all to remain seated because French police were going to board the aircraft to deal with ‘a serious security issue,’” he said.

In a video recorded by Jackson, the captain can be heard saying, “Folks, this is the captain, we are just waiting for the police to come on board. They may be here now and they directed us to keep everyone on the airplane until we sort out the extra passenger that’s on the plane.”

Jackson said the flight was completely full and there appeared to be no extra seats for the stowaway.

“I overheard the flight attendants talking about it with the pilots - they said this person was in one lavatory and then would exit and walk to a different lavatory and go in there for a long time,” Jackson told CNN.

The stowaway is between 55 and 60 years old. She has a Russian passport and will soon be sent back to the United States, according to an airport official in Paris.

The woman is currently in a waiting zone at Charles de Gaulle airport - known as ZAPI - for people awaiting deportation, as she does not meet the conditions for entering Europe. The woman applied for asylum in France a few years ago, the official told CNN.

When asked, the source did not address the woman’s mental state. CNN has reached out to the FBI on the matter.

“Nothing is of greater importance than matters of safety and security,” a Delta spokesperson said in a statement. “That’s why Delta is conducting an exhaustive investigation of what may have occurred and will work collaboratively with other aviation stakeholders and law enforcement to that end.”

The incident happened on one of the busiest travel days of the year. Nearly 2.7 million passengers travelled on airplanes on Tuesday, according to TSA data.

A former commercial pilot emphasized the seriousness of the security breach. “Even stand-by passengers just don’t get on board. You have to go through the gate agent,” JP Tristani told CNN affiliate WABC. “There’s a failure here that, frankly, 33 years flying worldwide airlines, that is inconceivable to me ( … ) We have a gross failure here of security right on down the line.”