It is a Friday the 13th that Paul Meechan will not forget soon.

On that night last week, bioterrorism researchers at the US Centres for Disease Control and Prevention (CDC) discovered they had mistakenly sent live anthrax virus out to fellow scientists in two lower-security clearance labs at the agency, instead of what they thought were harmless samples of the deadly pathogen.

The initial safety lapse occurred in the CDC’s Bioterror Rapid Response and Advanced Technology laboratory, a high-security lab that was trying out a new protocol for inactivating anthrax, using chemicals instead of radiation.

In an interview, the CDC’s Meechan described some of the events that led to the discovery that as many as 75 agency staff had been exposed to live anthrax. The CDC first disclosed the incident to Reuters on Thursday.

The scientists in the Bioterror Rapid Response units had been preparing an especially-dangerous strain of the bacteria for use in two lower-security CDC labs, the Biotechnology Core Facility and the Special Bacteriology Reference Laboratory, Meechan said.

Bioterror lab team used a new process to purify samplesthey were not very experienced with

Those teams were experimenting with methods to more quickly identify anthrax in substances and powders sent to the US.

“If there was a bioterrorism incident, we could more quickly identify yes or no, this sample has anthrax,” said Meechan, director of the CDC’s environmental health and safety compliance office. He said the team in the bioterror lab used a new process to purify anthrax samples that they had not had a lot of experience with. To check their work, they took a sample of what they thought was dead bacteria and put it on a nutrient-rich lab dish called an agar plate to see if the bacteria would grow.

“They waited 24 hours. They took a look at the plate and they didn’t see any new growth,” Meechan said. “At that point they assumed the material was safe.”

Researchers took the slides to the two lower-security CDC labs which were trying to develop the new tests. Their experiments did not work and a week later, one of the labs asked for additional inactivated samples.

At the time, researchers in the bioterror lab discovered that they had left the agar plates in an incubator for an additional week, Meechan said.

As they were about to dispose of them, they noticed growth on one of the agar plates. “The growth turned out to be anthrax,” he said.

That is when the scientists realised the samples they had sent to the two lower-security labs may have contained live anthrax bacteria. People working in those labs take fewer safety precautions and were unlikely to be wearing a respirator, putting them at higher risk for infection.

Meechan said the team immediately pulled back the samples and contacted the staff members who had handled them.

That was on the evening of Friday, June 13.

Meechan said they reached some of the lab workers that same night. Since then, they have been interviewing managers and using electronic surveillance and key card data to identify anyone who might have been inside one of the two labs testing the samples.

The CDC has reached out to all identified individuals, who have been offered antibiotics and a vaccine.

No instances of illness have been reported.

Prior anthrax, ricin scares in the US

As many as 75 US government scientists may have been exposed to live anthrax bacteria after failing to follow proper safety procedures, prompting an investigation by federal authorities.

Below are previous incidents involving such dangerous organisms.

• The most prominent anthrax attacks in the US came by mail in 2001, shortly after the September 11 airliner attacks on New York and Washington.

Letters containing anthrax spores were mailed to two Democratic senators – Tom Daschle and Patrick Leahy – and to three TV networks and two newspapers. Five people died and 17 others were sickened by contact with the tainted mail. Two of the dead were postal workers infected while processing the mail.

The leading suspect, a US scientist named Bruce Ivins, committed suicide in 2008, before he was charged with any crime.

• In 2003, an anthrax scare led the US Postal Service to close 11 post offices in Washington DC, Maryland and Virginia after air quality monitors detected possible traces of anthrax spores at a Washington facility.

• The postal system has also been used to deliver ricin, another potentially-deadly toxin. In 2004, an envelope containing ricin was discovered in Senator Bill Frist’s mailroom in Washington, DC.

• In April 2013, an envelope addressed to President Barack Obama mailed from Memphis, Tennessee tested positive for ricin.

A second letter also from Memphis and intended for Mississippi Senator Roger Wicker was discovered at a mail processing facility before it could reach its intended target. It also tested positive for ricin.