Seven staff were arrested in a police raid on the Lagos offices of The Nation newspaper on Tuesday.
The action is thought to be linked to the publication of a letter that was apparently sent by former Nigerian president Olusegun Obasanjo to the current president Goodluck Jonathan.
Of the seven - six journalists and the head of company security - six were released after a lengthy interrogation without charge.
The managing editor of the paper's northern division, Yusuf Ali, is still under detention.
The arrests were condemned by the Newspapers Proprietor Association of Nigeria, the International Press Institute, the Nigerian Guild of Editors and Amnesty International.
Sources: The Nation/IPI
