The Times reports that FBI agents and some top officials became suspicious of Trump's ties to Russia during the 2016 presidential campaign but didn't launch an investigation at that time because they weren't sure how to approach such a sensitive and important probe, according to the sources. But Trump's behavior in the days around Comey's May 2017 firing, specifically two instances in which he seemed to tie Comey's ousting to the Russia investigation, helped trigger the counterintelligence part of the investigation, according to the Times' sources.