They have a page dedicated to everything they have given over to the US government: signal.org/bigbrother/

Signal themselves cannot decrypt the messages, so they are literally not able to provide any substantial information to the government. They can literally only provide two timestamps, when the user registered and the last time they connected to the server.

the only information we can produce in response to a request like this is the date and time a user registered with Signal and the last date of a user’s connectivity to the Signal service.

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