A spokesperson for the CDC said in a statement on Monday that the agency “may never know” exactly how a harmless strain of the bird flu managed to be cross-contaminated with a much more harmful strain before being sent to a USDA lab.

This statement was released as part of an investigation into the safety practices of CDC employees. The investigation began after anthrax had been erroneously deemed inactive before being mishandled in a way that exposed 80 employees to possible contamination.

Five total incidents regarding improperly handled anthrax or bird flu samples have been investigated. However, Dr. Thomas Frieden, director of the CDC, has said that sending the cross-contaminated sample to the USDA was “the most distressing” incident because it has risked public exposure to the virus.

The mistake was reported to the CDC in May, when the USDA lab found that all of their chickens had died during testing with the samples thought to contain the harmless virus. Reports did not make it to the higher levels of the CDC personnel for another six weeks.

Investigations into the delay of the report as well as how the contamination occurred are turning up empty. The persistence of the mystery is largely thought to continue because the materials used to cultivate the virus samples have all been discarded. Cross-contamination can occur when instruments are not sanitized properly or when a growth medium is accidentally used that is contaminated or infected.

The investigation into this incident ended on Friday, and despite the lack of answers several actions have been taken to ensure the same problems do not occur in the future. Two labs in the CDC that work with anthrax and bird flu have been closed, and no additional samples will be sent from the labs until safety protocol is secured. A panel of safety experts from outside the CDC is being formed to evaluate and advise on procedures within the labs.