Something very strange happened in Tuen Mun on October 28th, 2019. Starting around 4:00 pm, residents of the Tai Hing neighborhood reported both smelling tear gas and showing common tear gas symptoms. Photos collected by Apple Daily from the scene show both residents and firefighters responding to something.

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Residents and netizens quickly came to the conclusion that HKPF was either ‘testing’ new tear gas at their Tai Hing Operations Base or that tear gas had ‘leaked.’ After HKPF denied having anything to do with the incident, residents surrounded the Operations Base demanding answers. They were promptly tear-gassed. Somewhere between 70 and 80 people were arrested for similar protests over the next few days.

The people arrested have never been provided a satisfactory answer about what happened on the afternoon of October 28th. It was a question worth exploring for OSINT HK in the days when we were just a hashtag on Twitter. To start with, neither the ‘test’ or ‘leak’ hypothesis added up. For one, Tai Hing Operational Base has no outdoor area to test tear gas. It’s extremely unlikely they were firing tear gas in their small parking lot (and even less likely that no one witnessed it).

As to the ‘leak’ hypothesis: that’s not how tear gas works. The HKPF Black Flag warning of ‘tear smoke’ is actually a far more accurate description than a term like ‘tear gas.’ CS is a solid that releases irritants when it combusts. Different types of accelerants are used to start the process, which is one reason the Chinese tear gas burns so much hotter than the American gas used at the beginning of the movement.

The only way for tear gas to have ‘leaked’ from Tai Hing Operational Base would have been for tear gas rounds to have been set off the same way we see on the streets: being fired from riot guns. Either that or the munitions were somehow accidentally ignited to the same effect. Once again, we think residents would have noticed tear gas rounds bursting inside a police station. It would likely have caused a fire inside as well.

None of which residents reported. Not even smoke coming from the Operational Base. Both theories were based entirely off the Operational Base’s proximity to reported smells and effects and the association of tear gas with the Hong Kong Police Force.

Ruling out the popular theories leaves two remaining: either the tear gas came from somewhere else or it was a psychosomatic mass hysteria.

Where might the tear gas have come from? The same place OSINT HK geolocated when we wondered where the PLA Hong Kong Garrison was firing anti-tank missiles in a video they released in August. Directly across Castle Peak mountain range from Tai Hing is a massive PLA firing range. The same video also showed PLA soldiers practicing firing tear gas and using other riot control tactics on a mocked-up Hong Kong street (one of two scenes we haven’t been able to geolocate yet).

What’s more, a marketing video trying to sell Chinese crowd control equipment to Venezuala shows released a few months before this incident shows vehicles blanketing a street with far more tear gas than HKPF can manually fire. They’ve already filmed themselves firing cannons, anti-tank missiles, and sniper rifles at their Castle Peak Firing Range… why not a massive amount of tear gas?

Weather reports from Hong Kong International Airport showed a sudden uptick in wind speed at around the time of the incident. Wind speed doubled to 21 kts/10.8 m/s compared to earlier in the day. A simple diagram map showed that the wind would have been blowing in almost a straight line from the Castle Peak Firing Range to the Tai Hing neighborhood of Tuen Mun.

Nathan Ruser then took that data and plugged it into NOAA’s HYSPLIT particle dispersal model program. It was a perfect match.

Can OSINT HK say for certain that the PLA accidentally gassed Tuen Mun? No. But the police denial of responsibility matches our own debunking of the two theories of where the gas came from. Given that as many as 80 people were arrested because of their outrage over the incident and neither HKPF or the HKSAR government providing any answers, it is also unlikely that the PLA Garrison would admit to having triggered this bout of unrest.

Ultimately, the only two theories that stack up to scrutiny are mass hysteria or the PLA accidental gassing scenario.