A section of Apple’s Cupertino campus was evacuated on March 15 after some suspicious white powder was found in an envelope on the premises. The local Santa Clara County Fire Department responded to a raised fire and hazmat alarm in Apple Park around 12 noon after first responders found a suspicious substance in an envelope, reported NBC Bay Area.

Employees were cleared to head back inside roughly six hours after the incident when it was determined that the situation was under control. The fire department has not revealed what the substance was.

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Apple told its employees at the campus that authorities concluded that there was no presence of hazardous material, according to an email obtained by the Verge.

The incident harkens back to 2001 soon after the 9/11 terrorist attacks when letters containing anthrax spores were received by several media houses as well as Democratic Senators Tom Daschle and Patrick Leahy. The incident resulted in the death of five individuals, and infection in 17 others.

Anthrax is a deadly infection caused by the bacterium Bacillus anthracis, which can be spread through the spores of the bacteria that can be ground up in white powder. The letter had been coated with concentrated anthrax spores, which can be cultivated with minimal special equipment and minimal collegiate knowledge of microbiology.

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(Edited by : Shoma Bhattacharjee)