The 9-year-old boy who stowed away on a Delta flight from Minneapolis to Las Vegas on Thursday passed through three security checkpoints at the airport without a boarding pass or identification, officials and an airline expert said Sunday.

“I’ve worked at the airport for 13 years, and we have more than 33 million people go through the terminal every year, and I’ve never heard of anything like this happening before,” said Pat Hogan, a spokesman for the Metropolitan Airports Commission.

The boy got through the Transportation Safety Administration (TSA) checkpoint and past a Delta gate agent, and didn’t get scrutinized by flight attendants before the plane took off from Minneapolis-St. Paul International Airport, said Terry Trippler, owner of ThePlaneRules.com.

“I put it more on Delta than the TSA,” said Trippler, saying that the boy blended in with a family traveling with children. If an adult handed the TSA agent six boarding passes, it would be fairly easy to miss it if there were seven people, he said.

“The kid’s smart,” Trippler said. “I’m going to give [the TSA agents] a little break. The way I look at it is, ‘Delta, this is a person getting on your flight. You make sure they’re all there.’ A lot of people want to come down on the TSA. I could see where it could happen. I could see where it could happen with Delta, too, but it really shouldn’t happen with Delta.”

Delta spokeswoman Leslie Scott said Sunday that the incident is under investigation. The airline “is working with the various authorities involved,” she said.

The TSA, in a statement to news organizations Sunday, said it was reviewing whether checkpoint changes were needed. As for Thursday’s incident, the TSA said: “The child was screened along with all other passengers to ensure that he was not a threat to the aircraft.”

The boy, who has not been identified, actually went to the airport both Wednesday and Thursday via light-rail, Hogan said Sunday.

On Wednesday, the boy took a bag off the baggage carousel, went through TSA security and had lunch at a restaurant in the area that leads to the concourses. He told staff at the restaurant that he had to use the restroom. Since there isn’t a restroom in the restaurant, the boy had to enter one of the concourses. He left the bag and his lunch bill behind and never returned, Hogan said.

The bag belonged to an arriving passenger. When the passenger arrived, airport personnel were waiting for him and returned the bag. The passenger reported that nothing was missing.

On Thursday, the boy was seen on surveillance video talking to a Delta agent at the gate, Hogan said. While that agent was busy, the boy walked down the jetway and onto the plane. It’s unclear whether he chose the flight at random or specifically wanted to get to the gambling mecca.

The flight was not full and there were empty seats, Hogan said. The flight crew became suspicious midflight because the boy was not on their list of unattended minors and didn’t appear to be traveling with an adult.

Las Vegas police were notified and took the boy into protective custody when the flight landed. Las Vegas police called Minneapolis police, who took a missing child report and went to the boy’s home to inquire about the boy’s whereabouts. The family said he hadn’t been seen since Thursday morning.

Minneapolis police Sgt. Bill Palmer said Hennepin County Child Protection and airport police are investigating. Hogan said it is not clear whether someone flew to Las Vegas to retrieve the boy or whether he was put on a flight back to Minneapolis.

Trippler said the incident “doesn’t bolster your confidence in the security system.” “It shows a situation where these people are human and they’re going to make mistakes.” He said the bigger message is if passengers see something, say something. “It tells us passengers, always heads up,” he said.