Well, both.
Warrants may be granted very easily via the FISA courts [0]. They are unenforced in the warrantless wiretap programs [1]. If your wiretap happened to be illegal, and you want to work around this to win the court case anyway, you use parallel construction to build a case that does not rely on the secret wiretap [2]:
> Because the SOD's work is classified, DEA cases that began as NSA leads can't be seen to have originated from a NSA source.
> So what does the DEA do? It makes up the story of how the agency really came to the case in a process known as "parallel construction."
[0]: https://en.wikipedia.org/wiki/United_States_Foreign_Intellig...
[1]: https://en.wikipedia.org/wiki/NSA_warrantless_surveillance_(...
[2]: https://www.washingtonpost.com/news/the-switch/wp/2013/08/05...