That's just standard cop stonewalling. They'll give out the descriptions when they're good and ready, along with other info. Also, if as sounds likely they've either arrested or are actively chasing perpetrators, so that may also be a factor. Suppose they've got one say, and are chasing three.
Who benefits if they say "we've got Ahmad Mohammad, 27" to gratify reporters if the others are still loose. Better if they stay in the dark.
"They'll give out the descriptions when they're good and ready, along with other info"
...meanwhile the perps have boarded a plane and are on their way to who knows where. Why not give a description while its early enough in the game for some citizen to recognize them? (..unless they really have no good descriptions at this point--quite possibly the case!)