Analysis: James Copnall, former BBC Sudan correspondent
The National Intelligence and Security Service (NISS) is an extremely powerful body, which frequently intervenes in Sudanese politics.
It is a key part of the informal coalition - also comprising the military, Islamists and pragmatists - which rules Sudan.
The different components are constantly jockeying for a better position.
In recent times, NISS has been flexing its muscles.
It is very possible that NISS did not like the decision to release Meriam Ibrahim, and re-arresting her and her family was a way of making this point to the rest of the Sudanese government.
However, security is not a homogenous entity either.
It is also conceivable that one part of NISS accepted Mrs Ibrahim's release, while another section was not happy with it.
Mrs Ibrahim's release and re-arrest simply underline the fact that there are many decision-makers in Sudanese politics, and they do not always agree with each other.