(CNN) -- A lab mix-up involving a deadly flu virus strain added to the Centers for Disease Control and Prevention's public relations nightmare this summer.
On Friday, the public health agency released the results of its internal investigation into the incident, along with a series of reports on actions being taken to ensure lab safety protocols are being properly followed.
In May, CDC lab workers unintentionally mixed a low pathogenic, or non-dangerous, avian influenza virus sample with a potentially deadly virus called H5N1, then shipped the sample to an external lab run by the U.S. Department of Agriculture.
The H5N1 strain has killed millions of birds and infected more than 600 people over the last decade. But no one appears to have been at-risk of being infected during the incident since the sample was stored and shipped properly, according to the CDC reports.
"The contamination most likely happened due to the failure of a laboratory scientist to adhere to established best practices and the absence of an approved laboratory team-specific protocol for the work being done," the reports concluded.
Dr. Tom Frieden, the CDC's director, found out about the H5N1 incident nearly six weeks after it happened, something he called at the time "most distressing." This delay in reporting the cross-contamination was due to "a lack of awareness and understanding about reporting requirements for Select Agents," according to the reports.
Around the same time he heard about the flu incident, Frieden was dealing with the possible exposure of up to 86 of his workers to anthrax.
Michael Farrell, who headed the Bioterrorism Rapid Response and Advanced Technology Laboratory involved, submitted his resignation in July. He had been reassigned from his post in June after the anthrax incident was made public.
The CDC says the potential anthrax exposure happened between June 6 and 13. The bioterrorism lab had been preparing anthrax samples for use in two other labs on the CDC's Atlanta campus and failed to deactivate the samples adequately.
According to another CDC investigation report, the exposure happened because the lab didn't use an approved sterilization technique. It didn't have a written plan reviewed by senior staff to make sure all safety protocols were followed, and there was a limited knowledge of peer-reviewed literature about the process that would make it less dangerous. The lab also did not have a standard operating procedure that would make sure the transfer of the material would be safe.
Bottom line: "The scientists failed to follow a scientifically derived and reviewed protocol that would have assured the anthrax was deactivated," Frieden said last month. It "should have happened, and it didn't."
As a result of these problems, the CDC has stopped transferring materials in and out of its highest-level labs. "The moratorium will remain in place pending lab-by-lab review of policies and procedures for laboratory safety and security," the CDC says.
Frieden plans to appoint a CDC director of laboratory safety, and establish a working group to review safety protocols.
"The work of safety is never complete," Dr. Michael Bell, interim director of laboratory safety, said in a statement. "We will continue to take all necessary steps to maintain safety and advance CDC's critical laboratory science work, so that CDC labs are the most scientifically rigorous and the safest in the world."
CNN's Elizabeth Cohen and Jen Christensen contributed to this report.