There are two situations in which the statute of limitations may be extended or tolled.
Application for Overseas Evidence
The government may seek to toll the statute of limitations for up to three years if it has made an ex parte application to a court based on an outstanding request for evidence outside the US.1 The limitations period is tolled from the date of the official request for evidence until the date the foreign country takes “final action” on the request. Tolling cannot be used to revive charges once the statute has already expired.
The government frequently requests that the subjects of a potential enforcement action agree to toll the statute of limitations, typically for six- or 12-month periods. Tolling agreements may be renewed indefinitely by mutual agreement of the parties.
1 18 USC § 3292.