Admitted by the Chairman of the House Intelligence Committee, Nunes. There's no investigation because it wasn't illegal, obviously. Obama wouldn't get people to illegally wiretap anyone. He doesn't need to.
Trump and his associates weren't ostensibly the target, they collected information on Trump "inadvertently." Common tactic, get a wide data set and then focus on those you want afterwards while having plausible deniability that you were focusing on the person you were. Look up parallel construction.
It was a result of the latter FISA wiretap:
http://www.nationalreview.com/article/443768/obama-fisa-trump-wiretap