nbc s tom costello covers aviation for us and joins me this morning. good morning. what do we know? reporter: we have five different aircraft today that we are told were affected by a bomb threat threats called in have all been deemed to be not credible, and four of the five flights were already on the ground safely. american 648, san diego to philly delta flight 55 los angeles to atlanta, united 995, which was san francisco to chicago, and you re looking right there at the scene of philadelphia, by the way, that s american 648, san diego to philly. last flight 939, portland to guadalajara, mexico also we re told on the ground. one flight still in the air, korean air 23 seoul, south korea to san francisco. that flight is still in the air, landing 2:20 p.m. eastern time in san francisco. that threat deemed not credible. korean air is continuing on with the flight. all of these threats are, we are told, very similar to the ones that were phoned in over the past couple of week