Image credit: Obtained by ABC News

Authorities today continued to investigate how a man was able to board a Los Angeles-bound plane at San Diego International Airport Tuesday without a ticket.

The man, whose name has not been released, was arrested and charged with trespassing. He'd just been released from prison and authorities said he had an extensive criminal history.

The suspect allegedly walked through an alarmed but unguarded emergency door inside the terminal. He then joined a line of passengers boarding United Airlines flight 6323 on the tarmac.

"When he went through the door, it set off the alarm,"  San Diego Harbor Police Lt. James Jordan told Fox 5 San Diego.  "It was caught on camera."

Flight attendants became aware of the extra flier, who was then seated on the plane, during a final passenger count.

"They said: 'What's your count?'" said passenger Nicholas Blasgen. "She said: 'This is my count.' And they said: 'That's wrong.'"

Agents from the Transportation Security Administration arrested the man who was reportedly not deemed a threat.