It depends on the type of shoe. By the time investigators arrived there were several firemen and first responders who had walked through that room fighting the blaze and recovering the bodies, leaving numerous footprints. Police probably zeroed in on one print that didn't match that typically worn by firemen. (Note they likely collected boots from firefighters at the scene.) Police then noticed that the print on the door was of the same type as the one outlier from the bedroom. Now roll the video of the arrest please. Look closely at the shoes worn by the perp. in the dark blue hoodie. To me they appear to be camel or beige in color. Could be Timberline brand which has a tread pattern similar to the foot print on the door but also similar to thousands of other popular boots. Probably not enough evidence to tie that man to the crime but adequate circumstance to follow his every move until he leads them to something big.
Fire did not reach the bedroom where adults were killed.
