Nigerian anti-corruption agents have charged two ex-aviation ministers after $160m for radar equipment went missing.
Femi Fani-Kayode and Babalola Borishade denied misappropriating money from an emergency fund set up to improve air safety after a series of fatal crashes.
Following the arrest of the two and a third official on Monday, the Senate passed a resolution to stop witnesses at parliamentary probes being detained.
Senators argued that the arrests could frighten off other potential witnesses.
Nigeria's parliament is conducting multiple investigations into alleged massive corruption during the government of former President Olusegun Obasanjo.
NIGERIA PLANE CRASHES
Roland Iyayi, the former managing director of the Nigerian Airspace Management Agency, was also charged.
In his testimony, Mr Borishade said that the amounts of money concerned had been exaggerated.
The equipment was supposed to help improve Nigeria's woeful air safety record after several fatal crashes.
Mr Borishade was sacked as aviation minister after a 2006 crash at the airport in the capital, Abuja, which killed 97 people, including the spiritual leader of Nigeria's Muslims.
Mr Fani-Kayode, a spokesman for Mr Obasanjo who left office last year, then replaced him.
Last week, an Austrian aviation contractor was arrested after giving evidence.
More than 200 people were killed in two crashes in 2005.
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