No matter how much China already restricts citizens’ online activities, the country is constantly introducing new ways to monitor and control its internet. Back in July, WhatsApp became the latest service to come under government scrutiny when users discovered they could no longer send photos and videos. Now, many people have found that the app has stopped working altogether.

Nadim Kobeissi, an applied cryptographer at Symbolic Software, a Paris-based research start-up that monitors digital censorship in China, told The Verge it was likely that China’s government had upgraded its Great Firewall to detect and block the NoiseSocket protocol WhatsApp uses to send texts. This is in addition to already blocking the HTTPS/TLS that the software uses to send photos and videos.

Stopping citizens from using WhatsApp, which features end-to-end encryption, could force them onto alternatives that share user data with the government, such as WeChat.

The move will come as a disappointment for WhatsApp owner Facebook. CEO Mark Zuckerberg has been looking for a way to enter the Chinese marker since the social network was blocked in 2009. With Instagram also unavailable, WhatsApp had been the last of Facebook’s available apps in the country.

The block comes in the runup to the Communist Party’s congress, which begins on October 18. The meeting will see the government choose its new leaders, though it is expected that President Xi Jinping will remain in charge.

July also brought news that China would start auditing online content to make sure it meets “core socialist values.”