Anne Donnelly, the district attorney for Nassau County, said the allegations that have surfaced in recent days regarding Santos were serious.
"The numerous fabrications and inconsistencies associated with Congressman-Elect Santos are nothing short of stunning," Donnelly, a Republican, said in a statement.
But reporting by the New York Times and other media outlets in recent weeks called into question almost every element of Santos' life story.
Among other claims, Santos said he had degrees from New York University and Baruch College, despite neither institution having any record of him attending.
In recent days, Santos has apologized for "embellishing" his resume, while defending aspects of the way he had represented himself.