Italian financial police on Tuesday raided the offices of Massimo Mantovani, a member of the Eni management and the group's former legal director, the sources said.
One of Eni's lawyers, Piero Amara, is also among those arrested.
Investigators are probing if the pair tried to throw Italian investigators off course in order to spare the Eni group and its general manager Claude Descalzi corruption allegations surrounding an oil contract in Nigeria.
Eni and fellow petroleum company Shell are due to stand trial in Milan next month over charges of bribery and corruption in the 2011 purchase of an offshore oil block in the west African country.
The companies are accused of graft in the acquisition of OPL245, estimated to hold nine billion barrels of crude, for $1.3 billion.
Among the defendants are Eni's current boss Descalzi, his predecessor Paolo Scaroni, as well as other executives and executives of Shell and the Italian group, as well as Nigerian ex-minister of oil, Dan Etete.
The Eni group confirmed in a statement the search in the offices and at the home of Mantovani, underlining its "confidence in the honesty of the actions carried out by its leaders".
"Eni, which is not under investigation, hopes that clarity will be made as quickly as possible," the group added.