Sure you can easily get further data by for example asking the phone companies for cell-tower log-in location and times. This you can then narrow down against your list of Signal using suspects and either remotely infect their phones with a trojan or simply snatch up the hardware at a “random” police check and access the already decrypted messages with identifiable phone-numbers of all the group-members.

What the fuck? Sure, you could also just being tortured till you tell them everything you know, but fking tracing over cell companies is not a security flaw in an app.

They could also just as well decrypt your self hosted emails that are cached on your device.