One thing to remember is that Tor was created by the government, specifically to encrypt the communication of CIA spies. They released it to the public specifically to provide plausible deniability to those spies. So backdooring the protocol right out of the gate would be an own goal.

That being said, I have no clue if they still use it. Presumably if the US government found a vulnerability in Tor, they'd either stop using it, or run some other layer of encryption on top.