I think the suspicion is NASA might have preliminary evidence of life but not the direct confirmation. If you find a planet with the right temperature, an atmosphere, liquid water and certain gasses evident, it tells you there is probably life. But confirming that 100% is the issue,
I think if/when they confirm microbial life, they will release it.
If they have evidence of intelligent life...that, Im not so sure about. I would suspect the US government would "National Security" that information into oblivion. The problem is, there are other nations that might not. Who knows.
I tend to think the discovery of life, even if its advanced would be a good thing for the world. Even the Pope "endorsed" aliens a few years ag.