Four buildings at Facebook’s postal centre in Silicon Valley had to be evacuated after a package tested positive for the presence of a deadly nerve agent.

Two people were assessed by medical experts for exposure to sarin when the alarm was raised following a routine check on an incoming parcel.

The social media firm's Menlo Park site was placed under quarantine at 11am local time (7pm BST) on Monday morning.

Three of the buildings were later cleared for people to return, and reports said that a subsequent test on the suspect package had indicated the initial result may have been a false positive.

“We have two possible exposures but right now they are not exhibiting any signs or symptoms,” said Menlo Park fire marshall Jon Johnston.

“The (Facebook) facility tests all of the packages that come in and they had a positive test, so they just initiated their standard protocol. Now we’re just waiting to verify whether that’s true or not.”

Facebook birthday: 15 defining moments for the social network Show all 15 1 /15 Facebook birthday: 15 defining moments for the social network Facebook birthday: 15 defining moments for the social network Facebook is born On 4 Feb, 2004, 19-year-old Harvard student Mark Zuckerberg launched a website called 'TheFacebook' from his dorm. Within 24 hours the college social network had more than 1,000 users Wikimedia Commons Facebook birthday: 15 defining moments for the social network Winklevoss twins sue Zuckerberg Within one week of launching, fellow Harvard students Cameron Winklevoss, Tyler Winklevoss and Divya Narendra accused Zuckerberg of stealing their idea. It would be four years later when the resulting lawsuit was finally settled Facebook birthday: 15 defining moments for the social network Open for business The social network finally opened it platform to everyone on 26 September, 2006. The move proved the catalyst in supercharging the site's already explosive growth PA Facebook birthday: 15 defining moments for the social network Billion-dollar bid Yahoo offered $1 billion to buy Facebook in September 2006 but Zuckerberg turned it down. 'I don’t know what I could do with the money,' Zuckerberg reportedly said. 'I’d just start another social networking site' Reuters Facebook birthday: 15 defining moments for the social network In the money In September 2009, almost five years since the site launched, Facebook turned a profit for the first time Getty Images/iStockphoto Facebook birthday: 15 defining moments for the social network Taking the lead Facebook overtook MySpace in 2010 to become the world’s most popular social network Facebook birthday: 15 defining moments for the social network Taking on the tech giants In 2011, Google launched its own social network that it hoped would knock Facebook from its perch. Despite its initial success, Google+ ultimately failed and will be shut down completely in 2019 Getty Facebook birthday: 15 defining moments for the social network Facebook goes public On 18 May, 2012, Facebook went public. The initial public offering raised $16 billion – the third largest in US history Facebook birthday: 15 defining moments for the social network Gobbling up the competition Facebook acquired Instagram in April 2012 for $1 billion, consolidating its position as the world's leading social network Reuters Facebook birthday: 15 defining moments for the social network One billion users On 4 October, 2012, Zuckerberg announced that Facebook had hit 1 billion users. 'If you’re reading this: thank you for giving me and my little team the honour of serving you,' he wrote in a blog post Getty Images Facebook birthday: 15 defining moments for the social network Expanding its empire In February 2014 Facebook acquired the messaging app WhatsApp for $19.3 billion REUTERS/Dado Ruvic Facebook birthday: 15 defining moments for the social network Two billion users In June 2017, Facebook passed the 2 billion user milestone REUTERS/Dado Ruvic Facebook birthday: 15 defining moments for the social network Privacy scandal On 17 March 2018, news broke that UK firm Cambridge Analytica had harvested data from around 87 million Facebook users for the purpose of political profiling in the build up to the 2016 US presidential elections Shutterstock Facebook birthday: 15 defining moments for the social network Record profits Despite the scandals and subsequent #DeleteFacebook campaign, Facebook posted record profits just before its 15th anniversary, the equivalent of $7.37 from each of its 2.32 billions users iStock/Independent Facebook birthday: 15 defining moments for the social network Unhappy users A study found that people are happier when they don’t use Facebook, adding to mounting evidence surrounding the impact social media has on mental health Rex Features

The county fire battalion chief said a similar alarm, proven to be false, has happened at Facebook before.

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Facebook spokesman Anthony Harrison said the company evacuated four buildings following the positive test and was cooperating with police in the investigation.

“Authorities have not yet identified the substance found. As of now, three of the evacuated buildings have been cleared for repopulation,” Mr Harrison said.

The FBI is assisting in the investigation, as is common in incidents such as this one.

The federal Centres for Disease Control and Prevention says sarin is a chemical warfare agent that is a clear, colourless, odourless and tasteless liquid. It can evaporate into the environment, prompting symptoms within seconds.

A drop of sarin on skin can cause sweating and muscle twitching, and exposure to large doses can result in paralysis and respiratory failure leading to death.

The CDC says people who are mildly exposed usually recover completely.