The hurdles to prosecuting collusion may be why the FBI is looking hard at the finances of various Trump aides, one legal expert said. Collusion may not be illegal, but money laundering, tax evasion, bank fraud and wire fraud certainly are.

Legal experts say it may be more likely that crimes could have been committed in covering up alleged collusion. It is illegal to lie to the FBI and to omit material information from security clearance forms, for example.