They are probing claims the centre-right UMP party — which Sarkozy now heads — should not have paid fines levelled against the former French leader after he broke spending limits on his failed campaign for the 2012 presidency.
The UMP is accused of using public cash to pay off Sarkozy's fines, and the party's former chief Jean-Francois Cope has been charged over the affair, as has former party treasurer Catherine Vautrin.
The former president appeared at the financial section of the main Paris court on Wednesday for questioning by investigating judges.
They placed him under the so-called "assisted witness" status — a French designation that comes halfway between being charged and just being a witness.
The status means that the person involved is deemed to be implicated in one way or another in a judicial probe, but is not directly accused of committing an offence.
However, he could still be charged at a later date.
Sarkozy's campaign is also accused of falsifying invoices for events and passing off some €18.5 million ($20 million) in spending as UMP party expenses.
Sarkozy denies any knowledge of, or role in, the apparent fraud which meant his campaign spent nearly 50 percent more than it was legally entitled to.
This is just one of several legal woes that risk derailing Sarkozy's comeback, which saw him elected to the head of the UMP last autumn ahead of another possible run for the presidency in 2017.
But they have so far had little impact, with the UMP winning a thumping victory in weekend local elections.