High ranking sources inside the Capitol Police Dept. tell TMZ the FBI is looking at several facts that stink of an inside job. For starters, the Bureau is puzzled by the ease with which the mob found its way to House Speaker Nancy Pelosi's office.

It's not off any main hallway or next to other Congressional offices ... in fact, the path to get to the entrance is maze-like and not open to the public like other parts of the Capitol. The rioters who took it over -- vandalizing it and stealing a laptop -- got there within 10 minutes of entering the Capitol.

We're told the FBI feels someone must have provided a roadmap for them to find it that quickly. Remember, the Capitol has been closed to the public since March due to the pandemic so it's not like these guys could have been casing it to plot their path.

As for suspects -- we're told the FBI is looking at Capitol Police officers and many other Capitol staffers, such as engineers, plumbers and janitors.

We're told another image raising eyebrows at the Bureau are those videos of a few police officers opening gates and removing barricades to give the hoard easy access to the Capitol.

Our sources say some officers could be charged with cowardice -- legally defined as failure to take proper police action. Additionally, we're told the rioters -- and anyone who aided them -- could also be charged for threats and menacing against Congress ... because the House and Senate were in session when the mob breached the building.

We're told the feds are also extremely motivated to find the people responsible for attacking Capitol PD officer Brian Sicknick ... who later died from his injuries.

We're told the FBI and the National Security Agency are using cell phone tower data, and state-of-the-art digital imaging systems to help ID people who were in the Capitol during the insurrection -- that includes rioters, Capitol staffers.