For more than a decade, U.S. military labs sent live anthrax to at least 51 laboratories in 17 states, the District of Columbia, and three countries, the Pentagon acknowledged Wednesday. And the list of live-anthrax recipients is likely to keep growing.

“We expect this number may rise,” Deputy Defense Secretary Bob Work told reporters Wednesday.

That’s because every single lot, or master collection, of bacillus anthracis that has been tested so far has came back positive for live, activated anthrax spores, the Pentagon said Wednesday. That the initial tests were all positive hints at a widespread crisis in which the Pentagon sent live anthrax to an untold number of recipients. And only 1 percent of the samples have been tested so far. There are another 396 military lots yet to go.

Wednesday’s announcement was the latest in a series of Defense Department announcements on its mistakenly shipped live anthrax, each delivery increasing the number of laboratories affected. Last week, the Pentagon acknowledged sending anthrax to at least 24 laboratories in 11 states, a U.S. military base in South Korea, and a commercial lab in Australia. At the time, the U.S. military said that at least 20 military personnel were receiving antibiotics as a cautionary measure.

Now, the number of impacted personnel is 31. And two Canadian laboratories received live anthrax, the Pentagon acknowledged Tuesday.

In an anguished briefing Wednesday at the Pentagon, led by Work, officials sought to reassure the public that it was not in danger; that the Pentagon and the Centers for Disease Control were conducting a thorough review; and that before shipping out anthrax, Defense Department labs had conducted quality assurance tests that were accepted practice by the scientific community.

But officials also repeatedly warned that more lots could come back positive. It can take as long as 10 days for officials to say for certain that a lot has tested positive.

The Center for Disease Control is conducting an investigation. And Work has called for an internal review to determine, in part, how the department’s labs could have accidentally sent around live anthrax. (The labs depend on the military to provide anthrax samples to use for detection, decontamination, and training efforts.)

But those anthrax samples aren’t supposed to be alive when they’re shipped. There are strict procedures in place that are supposed to prevent that from happening. Officials said Wednesday that they tested each sample to ensure its live anthrax spores had been killed, going so far as to issue a “death certificate,” according to the director of medical programs for the Defense Department’s Chemical and Biological Defense programs, Commander Franca Jones.

Jones said Defense Department labs take a 5 percent aliquot, or sample, from the prepared liquid anthrax and test it. After waiting an unspecified amount of time, the sample of anthrax is carefully packaged for shipping, Jones explained. Then she demonstrated the process for the reporters gathered.

Jones insisted that techniques were in line with accepted scientific standards. But Richard Ebright, professor of chemistry and chemical biology at Rutgers University, noted there is no one set standard for testing anthrax. Many private labs, for example, insist on an aliquot as high as 20 percent.

“The less you take, the less reliable it is,” Ebright told The Daily Beast.

Despite that, Ebright noted that percentage is not enough alone to lead to so many Defense Department tests to incorrectly come up negative. He added that the results so far suggested a widescale problem. Either such tests were not conducted, he said, or done so in a “fundamentally flawed manner.”

The crisis erupted May 23 when a Maryland lab notified the Pentagon and the Centers for Disease Control that it had received live anthrax from Dugway Proving Ground, which sits about 86 miles of Salt Lake City. All four of the live lots detected so far originated at Dugway, but Defense Department officials said they are testing lots from all of its labs that deal with live anthrax.

In his briefing, Work sought to minimize the health risks to the public, noting the quantities sent out were too small to affect most—and were not dried, like the anthrax in the 2001 attacks that killed five people.

But Ebright said there was a larger, indirect danger. The labs working with anthrax could have sent out the samples to subcontractors without informing the Defense Department. Therefore, “There is no way they can put a ceiling” on how many received anthrax, he said.

“It is a massive security breach,” Ebright added.

The Defense Department had established a website with what it promises will be regular updates on the anthrax. But the Pentagon has yet to reveal which labs received the live spores.