A Sudanese woman freed from death row on Monday has been detained with her family at Khartoum airport, sources have told the BBC.
Meriam Ibrahim was sentenced in May to hang for renouncing Islam, sparking widespread outrage at home and abroad.
About 40 security agents detained Mrs Ibrahim - along with her husband, Daniel Wani and two children - at the airport, the sources said.
A top Sudanese official has told the BBC she would be freed "soon".
Abdullahi Alzareg from the ministry of foreign affairs told the BBC's Newshour programme that Mrs Ibrahim had been arrested because she did not have the correct travel documents.
Although she is Sudanese, she was using emergency South Sudanese papers with a US visa, he said.
Her husband is a Christian from what is now South Sudan and has US nationality.
One of Mrs Ibrahim's lawyers, el-Shareef Ali, told the BBC that her legal team is being denied access to her.
She was released from prison on Monday after an appeal court annulled the death sentence imposed on her.
She was arrested in February, and gave birth to a daughter in prison not long after being sentenced.
The family has been taken to the headquarters of one of Sudan's security agencies, the sources said.
Before she was detained on Tuesday, Western countries had welcomed the decision to rescind the death penalty.
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 underlines the fact that there are many decision-makers in Sudanese politics, and they do not always agree with each other.
Born to a Muslim father, Mrs Ibrahim, 27, married Mr Wani in 2011.
Sudan has a majority Muslim population, and Islamic law has been in force there since the 1980s.
Even though Mrs Ibrahim was brought up as an Orthodox Christian, the authorities considered her to be a Muslim because that is the religion of her father.
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