Evanston police are investigating an early morning explosion that left one man dead Tuesday and led to the evacuation of residents and the closure of a middle school. Police secured the area around the Nichols Middle School and Fitzsimmons Park, where the decapitated body of an unidentified male and an explosive device were found shortly before 6 a.m. At Tuesday afternoon news conference, Evanston Police Commander Tom Guenther said authorities were taking the explosion “extremely seriously’’ but did not yet understand what the bomber’s intentions were. Police refused to identify the man who died and said they had not determined...