WASHINGTON — Virulent swine flu spread to 10 U.S. states from coast to coast Wednesday and swept deeper into Europe, extending its global reach as President Barack Obama mourned the first U.S. death, a Mexican toddler who had traveled with his family to Texas. Total American cases surged to nearly 100, and Obama said wider school closings might be necessary.

The World Health Organization said the outbreak is moving closer to becoming a full-scale pandemic.

Dr. Keiji Fukuda, the organization's top flu expert, told reporters in Geneva that the latest developments are moving the agency closer to raising its pandemic alert to phase 5, indicating widespread human-to-human transmission. That's just one step below level 6, a full-fledged pandemic.

In Washington, Homeland Security Secretary Janet Napolitano was questioned closely by senators about whether the U.S. should close its border with Mexico, where the outbreak apparently began and the casualties have been the greatest. She repeated the administration's position that questioning of people at borders and ports of entry was sufficient for now and said closing borders "has not been merited by the facts."

Dr. Richard Besser, the acting chief of the Centers for Disease Control, said in Atlanta that there are confirmed cases now in ten states, with 51 in New York, 14 in California and 16 in Texas. Two cases have been confirmed in Kansas, Massachusetts and Michigan, while single cases have been reported in Arizona, Indiana, Nevada and Ohio.

In a possible outbreak north of the Mexican border, the commandant of the Marine Corps said a Marine lieutenant in southern California might have the illness and 39 Marines were being confined on their California base until tests come back.

Marine General James Conway told a Pentagon briefing an initial test indicated the sick Marine — who was not identified — might have swine flu but his illness did not appear life-threatening.

Obama said he wanted to extend "my thoughts and prayers" to the family of a nearly two-year-old Mexican boy who died in Houston, the first confirmed U.S. fatality among more than five dozen infections. Health officials in Texas said the child had traveled with his family from Mexico, to Brownsville on April 4 and was brought to Houston after becoming ill. He died Monday night.

"This is obviously a serious situation," and "we are closely and continuously monitoring" it, Obama said of the spreading illness.