(CNN) -- Hours after being released from jail, a man walked through an emergency door at San Diego International Airport, onto the tarmac and sat down on a United Express plane Tuesday, according to San Diego authorities.
"He completely bypassed TSA screening," San Diego Harbor Police Chief John Bolduc said. "He was in a public area and went out an emergency fire door, which gave him access to the tarmac."
Marc Duncan, 38, was paroled from jail Monday night, according to San Diego County Sheriff's Department records. He had been serving time for theft.
After it was opened, the emergency door alarm sounded, and Bolduc said police were on site in four minutes, but by then Duncan had blended in with other passengers.
He allegedly boarded a 30-seat United Express aircraft operated by SkyWest, which was heading to Los Angeles, according to airline spokesman Wes Horrocks.
The flight attendant realized she had too many passengers, Nicholas Blasgen, a passenger on the plane told CNN affiliate KGTV. "They said, 'What is your count?' She said this is my count, and they said that is wrong."
The passengers got off the plane and their luggage was searched.
"They had us put all the bags out, they separated the bags by enough distance and had the dog go over everything," Blasgen said.
Duncan was identified and arrested.
He is being held in the San Diego jail and is scheduled to be in court Thursday.
"I still can't, in the world, understand how this happened," Blasgen said. "It sounds like they just have lax security or not enough management. Something was going wrong."
The Harbor Police chief said they will examine this incident and find out where to make security improvements.
"The guy did breach security, but he was caught," Bolduc said. "We have multiple layers of security built into our airports, as you know, and the backup systems were able to catch this guy."
"Security of airports is a shared responsibility, and airports and airlines are required to adhere to TSA-approved security standards." TSA spokeswoman Sterling Payne said in a statement. "TSA has initiated an investigation and if necessary, will take appropriate action."
CNN's Todd Sperry contributed to this report.