the 150 terrorists killed by US bombs in Somalia

"killing about 150 fighters who were assembled for what American officials believe was a graduation ceremony and prelude to an imminent attack against American troops and their allies in East Africa." Wait: so they were at a ceremony AND planning an imminent attack against US troops? Which is it? Or was the ceremony to plan attacks?  And if they were planning attacks, how did the US find out from the air? Did their facial expression show signs of an imminent attack?