Diebold said in May that the SEC was conducting an informal inquiry relating to its revenue recognition policy.

But in its quarterly report, the vendor says the SEC has converted the probe from an informal inquiry to a "formal non-public investigation".

Diebold says it is continuing to cooperate with the SEC.

The vendor has been struggling over the last two years against falling financial services revenue in North America and Europe and has incurred restructuring charges to cut costs and improve its profitability. Last month Diebold reported a 46% drop in second quarter profits after earnings were hit by these restructuring charges.