2 possibilities:
They arrested a moslem but they’ve gotta keep him quiet so we don’t learn anything about his network so we cannot disrupt it.
OR
They broadcast “We’re about to go arrest somebody!” to spook the perps into fleeing their homes into the arms of SWAT waiting outside in a van. Safer for SWAT, and also a good indication of guilt.
It very well can be the case. I hear a reporter on the radio who was furious that someone in the government leaked out the info the authorities were looking for one person. Even a journalist on Fox was quite angry that the info was leaked to the public. Have to wait to see how the investigation goes now that there are so many conflicting reports.
Or they are waiting for Obama to arrive in Boston tomorrow so he can announce it at the memorial service.