PARIS — Three allies of President Nicolas Sarkozy of France on Friday found themselves the focus of an expanding investigation into a 17-year-old case involving suspected corruption in the sale of submarines to Pakistan.

The inquiry has gathered momentum at a difficult time for Mr. Sarkozy, only seven months before he is to seek re-election.

The case, known here as “the Karachi Affair,” centers on kickbacks that investigators suspect were paid to secure the sale of three submarines to Pakistan in 1994 and then used to help finance the presidential campaign of former Prime Minister Edouard Balladur in 1995.

On Thursday, the police arrested one of Mr. Sarkozy’s allies, Nicolas Bazire, who was a witness at his wedding to Carla Bruni in 2008 and was also Mr. Balladur’s campaign chief in 1995. Mr. Bazire was accused of complicity in the misuse of public money.