Donald Trump and others encouraged, instigated and incited the riot at the US Capitol on January 6. Worse yet, Trump made sure there were no executive branch law enforcement resources deployed to protect those in the Capitol building (given that the US Capitol Police are an agency controlled by Congress, not the executive branch).
Trump, Don Jr., Rudy Giuliani and others incited the crowd to march on and, in a very real sense, attack, the US Capitol, putting EVERYONE in the building – janitorial staff, administrative staff, members of Congress and even Vice President Pence – at imminent risk of serious injury or worse.
There generally are three ways to initiate a criminal case: a probable cause arrest, an arrest warrant and/or a grand jury indictment. Here’s an explanation of the difference among the three and why an arrest warrant is the best approach at this moment in time for Trump and the others.