Would't want to racial profile anybody by actually giving descriptions of the suspects!
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.