Ask HN: Can the NSA force Signal to backdoor it's software?

I have been reading about the FBI's gag order that it placed on Signal a few years ago, which Signal successfully fought with the help of the ACLU and were eventually (many months after the gag order had been applied) able to legally speak publicly about the matter. [0]

Which got me thinking about other tricks that powerful 3 letter agencies might have up their sleeves.

Is it possible that the NSA could serve Signal with an NSL forcing them to backdoor their software? Just like they did with all of the other major tech companies with the whole Snowden/PRISM debacle?

The open sourced Signal protocol can be as trusted as the Queen of England, but if somebody is backdooring the software that everybody downloads and installs on their phone...

Thoughts?

[0] https://arstechnica.com/tech-policy/2016/10/fbi-demands-signal-user-data-but-theres-not-much-to-hand-over/