New cases of the novel coronavirus reported Sunday, including the second death from the virus in the U.S., raised fears of a wider spread of the disease, prompting federal officials to ramp up efforts to test for and fight the growing health threat.

Health officials are focused on a cluster of confirmed cases in Washington, including the two deaths. Those infections, and several others in states such as Oregon and California—many with no clear path to exposure—signal that there might be wider spread of the virus in some communities,...