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The investigation concerns a software licensing deal that eventually caused Lucent to restate earnings in 2000 by $125m (£107m).
Despite the latest investigation, the US Attorney's Office has told Lucent it is not the target of the investigation, but no further details were disclosed as to what is being investigated.
A spokeswoman for the Attorney's Office declined comment on the case.
Lucent is "co-operating fully with the US Attorney's investigation", said a Lucent spokewoman.