San Diego Harbor police responded to the San Diego International Airport Thursday morning after a security alarm was triggered, prompting a ground stop.

A spokesperson for Harbor police said the alarm went off in Terminal 1 after three passengers, who were beyond security lines, decided to leave “to smoke.” When they tried to re-enter the secure area through an exit-only door, the alarm was triggered, beginning a process to secure planes.

Nicole Hall, spokesperson for SAN, said the alarm notified airlines to shut their jet bridges, which connect the airport to planes. Operations have since returned to normal, Hall said.

A passenger on a Southwest Airlines plane from San Diego to Sacramento said their plane was boarded by law enforcement officers with dogs and a carry-on luggage was removed. The passenger said the plane was being deplaned and would need to be inspected by security again.

The Federal Aviation Administration said a ground stop was issued at about 7:15 a.m “due to security.” The ground stop, which affected inbound flights from nearby airports, was expected to last for two hours, but could be extended.

No other details were immediately available. This is a developing story. Details may change as information is released.