Meriam Ibrahim has been detained with her family at Khartoum airport after she was freed from death row on Monday.
Meriam Ibrahim, 27, was sentenced in May to hang for renouncing Islam, sparking widespread outrage in Sudan and abroad.
About 40 security agents detained Meriam Ibrahim – along with her husband, Daniel Wani and two children – at the airport.
According to a top Sudanese official, Meriam 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.
Daniel Wani is a Christian from what is now South Sudan and has US nationality.
Meriam Ibrahim was arrested in February, and gave birth to a daughter in prison not long after being sentenced.