The number of facilities that received shipments of live anthrax from a U.S. Army laboratory has grown to 68, to include institutions in 19 states, Washington, D.C., and now four countries, including the United Kingdom, according to the Defense Department.

The investigation into shipments of spores sent to laboratories across the country and globe follows the discovery May 22 that live bacteria were sent by the U.S. Army's Dugway Proving Grounds in Utah to a facility in Maryland.

While the list of labs has increased, the number of civilian and military personnel taking medical precautions to prevent an infection has remained at 31.

The Pentagon has not released the list of labs that received the samples. But its has been updating its website on the investigation daily, adding Pennsylvania to the list Monday and the U.K. on Tuesday.

Other countries that received the shipments include South Korea, Australia and Canada.

Pentagon officials say the general public is at "zero risk" for exposure and said the samples, which were shipped as liquids in low concentrations, posed minimal risk to laboratory technicians or those who handled the shipments.

The Defense Department distributes anthrax to commercial, academic and government facilities to be used in research and to calibrate biological weapons sensors.

But according to Defense Department officials, the radiation that was supposed to kill the anthrax in these shipments did not destroy all live spores and equipment designed to test for live spores following irradiation failed.

The investigation has shown that some live anthrax shipments were sent as early as 2005.

The Defense Department and the Centers for Disease Control and Prevention continue to investigate the extent of the problem as well as failures of safeguards designed to prevent shipments of the potentially deadly bacteria.