Mexico shut down schools, museums, libraries and state-run theaters across its overcrowded capital Friday in hopes of containing a swine flu outbreak that authorities say killed at least 20 people - and perhaps dozens more. World health authorities worried openly that the strange new virus could become a global epidemic. The US Centers for Disease Control and Prevention said tests show some of the Mexico victims died from the same new strain of swine flu that sickened eight people in Texas and California. Mexico put the confirmed toll at 20 dead, but 40 other fatalities were being probed, and at least 943 nationwide were sick from the suspected flu, the health department said. Scientists said the virus combines genetic material from pigs, birds and humans in a way researchers have not seen before.