In one story of alleged misdeeds, as ABC News previously reported, the FBI contacted a Navy veteran, Richard Osthoff, about a GoFundMe campaign Santos established to raise money for the veteran's service dog.
Santos established the GoFundMe account under the auspices of a charity, Friends of Pets United, and raised $3,000 to help Osthoff pay for surgery to remove a tumor from the dog, sources said.
But Osthoff told ABC News Santos did not come through with the money and ignored text messages about it. The dog, Sapphire, subsequently died.
These allegations are not included in Wednesday's indictment, however.
Santos insisted earlier this year he would serve out his term despite mounting controversies surrounding his past falsehoods, scrutiny of his finances, and multiple investigations.
Santos, who has admitted to fabricating parts of his biography, has denied any criminal wrongdoing.