From all I've read Trump enterprises broke the law with great enthusiasm prior to him becoming president. They played fast and loose with rules, regulations, laws and particularly tax requirements and statements. Remember how Trump famously said he was under audit before the election? We've read a number of stories of him, his family and his business overvaluing and under-valuing assets for tax purposes. And we know he did a lot of deals with the Russians. Both his sons have said that it was Russian investment which saved their company after all the banks stopped doing business with them.
Steve Bannon suggested that collusion wasn't what he'd be caught out on but money laundering. And, I would guess, helping these Russian oligarchs evade sanctions, as well.
These sorts of investigations take a while, but New York State has been doing them for some time now. Soon the Justice Department and IRS will be free to join in and exchange information. I have no doubt, given his lifelong attitude (taken from his father, who cheated on his taxes and lowered value estimates of buildings he passed on to his son) that Trump has broken numerous laws and can be convicted if the government or governments so choose.