Washington CNN —  

The FBI and the federal prosecutors in Illinois are investigating whether Rep. Aaron Schock broke the law in accounting for campaign expenses, according to people familiar with the matter.

Schock in recent days announced plans to resign, citing controversy surrounding allegations that he improperly accounted for travel and other contributions from donors and mileage reimbursements for a personal car he purchased with campaign funds. But the probe managed by prosecutors in the U.S. Attorney’s office in Springfield, Ill., means his legal troubles are just beginning.

The federal investigation is focused on at least three questions, according to people familiar with the inquiry, including the mileage reimbursements, travel that was paid for by donors and in-kind contributions from donors that were not reported.

The FBI is delivering subpoenas in Washington and Illinois, according to two people familiar with the investigation.

Investigators from the U.S. Postal Inspection Service and IRS are pursuing various parts of the Schock investigation, according to law enforcement sources.

A controversy that began with questions sparked by a light-hearted profile of Schock’s “Downton Abbey”-themed office ballooned into a full-blown scandal over the past month that caused the rising GOP star’s rapid fall from grace.

A U.S. law enforcement official says the investigation is at an early stage. Justice Department officials were monitoring published reports about Schock’s expense troubles and were taken by surprise when he announced his resignation, according to another U.S. law enforcement official.

RELATED: Aaron Schock resigns amid scandal

CLARIFICATION: This story has been updated to clarify the ownership of a personal car for which Rep. Aaron Schock allegedly sought improper reimbursement.