About 1 p.m. the alarm went off on a magnetometer at the security checkpoint for Terminal 2, airport spokeswoman Rosemary Barnes said.
"The passenger was not detained and proceeded to the boarding gate area," she said, adding that she did not know why or how the man avoided the normal security protocol of being held for further screening.
Airport authorities then searched for the man, including searching two flights that were boarding, but could find no one meeting the description, Barnes said.
"When they did not locate the individual in a few minutes, they made the decision to evacuate the boarding area," sending passengers back to the public area of both terminals to be re-screened, Barnes said.
The incident was similar to another security breach
Jan. 5, in which a man darted into Terminal 2's passenger exit, prompting a total evacuation of both terminals.
Just as in the earlier incident, Thursday's apparent interloper was not found by any of the four entities responsible for airport security: the Oakland Police Department, Alameda County Sheriff's Department, Transportation Security Administration and the airport's privately contracted security contingent.
"A couple of thousand (passengers) were impacted today," Barnes said.
By 2:30 p.m., both terminals had been cleared and searched, and evacuated passengers had been re-creened.
Some 25 to 30 flights were expected to be delayed by the incident some for as long as three hours, Barnes said, although she estimated the airport would return to normal by early evening.
Contact Erik Nelson at email@example.com or at 208-6410. Read his Capricious Commuter blog at InsideBayArea.com.