They investigated anyways because there were plenty of other reasons to suspect that Flynn was on the payroll of foreign powers, given that he was paid by the Russian state through their state run media RT in 2014 and accepted at least one paid speaking engagement from the Russian state in 2015 without seeking US government approval. Flynn later plead guilty to having lied to the government about some of these engagements (though he recently retracted his plea agreement). https://www.reuters.com/article/us-usa-trump-russia-flynn-id...