Emergency authorities across the region urged people to stay home, and a utility company cut off power to 100,000 of its customers near Denver.

A Reddit user provided information that helped identify Claudio Manuel Neves Valente as the suspect not only in the campus shooting but also in the murder of an M.I.T. professor.

The shift would mean fewer shots recommended for children. But a Danish health official found the idea baffling, saying the United States was getting “crazier and crazier in public health.”