If the insurrectionists had a bite to eat, a rental car or a pillow to lay on while in D.C. last week before storming the Capitol, the feds just might find out about it ... because they're digging through a laundry list of receipts to track down the culprits.
Federal law enforcement sources tell TMZ ... the FBI has gone well beyond just resorting to flight manifests to find the people they suspect took part in Wednesday's siege -- they're looking at any and everything the rioters might've put their name to while in town.
That includes sifting through restaurant bills, hotel ledgers and rental car records in the days leading up to and following the attack on the Capitol ... and linking that up with intel of their own.
Granted ... there are literally hundreds more who didn't stick out so clearly ... so there's a lot more work to be done in the weeks and months to come as law enforcement attempts to bring all these people to justice.
TMZ broke the story ... they're investigating certain Capitol police officers and others who work at the Capitol, because there's evidence this could be an inside job. Our sources say 10 to 15 cops are under the microscope and the investigation is far along.