While I applaud Signal's response I expect this entire event (subpoena and response) will be provided as one of the exhibits to congress by the Department of Justice to justify their request that it be unlawful to provide such services. The DoJ will say, "See, here is this horrible crime we are investigating and because this company chose to make it impossible for law enforcement, with a warrant and a subpoena to get it, the criminal is going to go unpunished and that will be on you because you refused to mandate lawful access to communications."

The Congressional response should be, "Do you have no other way of investigating these criminals?" "Could you not put an officer out to surveille them?", "Have you not seen the misuse that law enforcement has engaged in, with such capabilities? From petty revenge to stalking lovers who rejected them. Will you consent to mandatory surveillance of all law enforcement officers that is recorded and stored in a civil controlled repository so that officer conduct may be reviewed at any time?"

They won't say that of course. But they should.