The deadly nerve agent sarin, which can cause paralysis and respiratory failure, might have entered Facebook's headquarters in Silicon Valley, prompting the tech giant to evacuate four buildings on Monday.
Two people are being tested for possible exposure.
The deadly item was delivered to one of Facebook's mail rooms, and it was deemed "suspicious". Further investigation is underway.
Here's what went down.
Let's address the most important question: What is sarin?
A man-made nerve agent, sarin was first developed in Germany in 1938. It's odorless, clear, tasteless and easily dissolves in water. People can come in contact with sarin in either liquid or gas form.
Though sarin evaporates quickly, it is one of the most volatile nerve agents.
Exposure to sarin can lead to convulsions, paralysis, breathing problems, and eventually death.
Previously, sarin attacks claimed lives of innocents
Even a fraction of an ounce of sarin can mess with the central nervous system.
Last year, 40-150 people were killed in a suspected sarin gas leak in Syria.
In fact, one of the deadliest sarin attacks happened in 1995. Members of a cult executed a sarin attack inside Tokyo's subway killing 13 people. Over 6,000 people fell ill at the time.
So, how did sarin enter Facebook?
The US Postal Service delivered a package to a mailroom on 1000 block of Hamilton Avenue. Following standard procedure, Facebook conducted tests and the package was found positive for "some kind of sarin".
Thereafter, Menlo Park Fire District sent a hazardous materials team to investigate the item.
Two employees who handled the package were also evaluated, said District Fire Chief Harold Schapelhouman.
The employees haven't shown any medical problems
"They don't have any medical problems. They are not complaining of any related effects that would lead us to believe in fact there was sarin of any shape or form involved," Schapelhouman added. The FBI is heading the investigation.
Safety of employees is our top priority: Facebook
Further, Facebook in a statement said the company would not compromise with the safety of employees.
"Authorities have not yet identified the substance found. As of now, three of the evacuated buildings have been cleared for repopulation. The safety of our employees is our top priority and we will share additional information when it is available," said spokesperson Anthony Harrison.
This incident shows tech companies and their employees aren't safe
The latest incident shows tech companies need to take the security of their employees seriously.
To recall, in April 2018, a woman killed three people at YouTube headquarters. She was reportedly angry with how the platform handled her videos.
Months later, in December, Facebook received an anonymous bomb threat after which its main office was evacuated. The threat later turned out to be a hoax.