A very serious question is whether Trump took action to hinder law enforcement at the Capitol.
He was involved in some degree in planning for the rally because he encouraged people to come and was even a speaker. News reports before the rally say that the permit for the rally was for a "March for Trump" rally at the Ellipse. That permit specifically said that although it was titled as a "March" that there would be no march to the Capitol or anywhere else.
Trump spoke at the rally and called on the mob to march to the Capitol. That raises the question of whether Trump may have helped organize the event to occur at the Ellipse, about a mile away from the Capitol, so that law enforcement would concentrate on the Ellipse and not the Capitol.
There were almost no Federal forces at the Capitol: Park Service, FBI, ATF, etc.
Requests for DC National Guard took about 90 minutes to get even approved by the Pentagon. Virginia rapid response teams were held at the border waiting for Pentagon approval to go in to DC.
Trump made many changes to positions of Pentagon officials over the last few months. That raises the question of whether he did that to have people in place to ensure that the Capitol would not be properly protected.
That is especially disconcerting because the previous Secretary of Defense was terminated by Trump when it was determined that he was a loser. He even raised concerns that there may be an attempted coup and that the Acting Secretary my be complicit with attempt.
Now I feel like a crazy conspiracy theorist. But these are issues that need to be investigated.