FBI started investing trump campaign russia ties when an Australia reported to USA that Popadopolus was talking to an Australian diplomat about his Russia activities. <---- that is the justification for starting it.
I think they started investigating the Russians specifically on this, when the Clinton emails were stolen and when our western allied Nations that we share intelligence sent us intelligence, which was part of what they had on the Russians as well...
the Trump campaign was added and their counterintelligence on the Russians who are always watched, got joined with the possible criminal acts of the Trump campaign, with the Carter Page Trips to Moscow and the Papadopolous incident when he was blabbing about the Russians having dirt on Clinton, stolen emails.... back in May 2016.....
which the Ambassador relayed to us a month or two later...when it came out in the open that the DNC emails were stolen.
the fbi fished around a little to see if any of the info they were receiving fro allies had teeth, and then late July, added the Trump campaign workers in a criminal investigation to the Russian counter intelligence investigation.