What makes you think that a crime MUST have already been committed in order for surveillance to be initiated?
The FBI started a counter-intelligence investigation on Trump's campaign because they received information from one of the Five Eyes co-operating allied foreign intelligence services who were spying on the Russians.
During their own routine "SIGINT" surveillance of The Kremlin and Russian oligarchs, they noticed multiple members of the Trump campaign and other Trump associates (e.g. Rick Gates, Paul Manafort, Don Jr, Ivanka, Jared Kushner, Mike Flynn, George Papadopoulis, Michael Cohen, et al) kept repeatedly popping up. They became concerned about that because non-Russian, non-Government citizens who keep turning up in surveillance on the Russian government almost always turn out to be Russian spies. As the UK is part of "Five Eyes" it was their duty to inform US government intelligence services such as the FBI about what they had observed.
The FBI/CIA would be derelict in their duty to the security of the United States if they had ignored such information.
Britain’s spy agencies played a crucial role in alerting their counterparts in Washington to contacts between members of Donald Trump’s campaign team and Russian intelligence operatives, the Guardian has been told.
GCHQ first became aware in late 2015 of suspicious “interactions” between figures connected to Trump and known or suspected Russian agents, a source close to UK intelligence said. This intelligence was passed to the US as part of a routine exchange of information, they added.
Over the next six months, until summer 2016, a number of western agencies shared further information on contacts between Trump’s inner circle and Russians, sources said.
The European countries that passed on electronic intelligence – known as SIGINT – included Germany, Estonia and Poland. Australia, a member of the “Five Eyes” spying alliance that also includes the US, UK, Canada and New Zealand, also relayed material, one source said.
The Guardian has been told the FBI and the CIA were slow to appreciate the extensive nature of contacts between Trump’s team and Moscow ahead of the US election. This was in part due to US law that prohibits US agencies from examining the private communications of American citizens without warrants. “They are trained not to do this,” the source stressed.
“It looks like the [US] agencies were asleep,” the source added. “They [the European agencies] were saying: ‘There are contacts going on between people close to Mr Trump and people we believe are Russian intelligence agents. You should be wary of this.’
“The message was: ‘Watch out. There’s something not right here.'”
https://www.theguardian.com/uk-news/2017/apr/13/british-spies-first-to-spot-trump-team-links-russia