Robert Mueller’s special-counsel investigation has dragged on for nearly two years — but it was rooted in FBI’s own probe of Trump campaign, a counterintelligence operation that looked back to early 2016.

Before he won GOP nomination, President Trump’s unconventional campaign took on a range of lightly vetted associates and volunteers whose activities caused alarm for US intelligence contacts in Australia and UK, eventually sparking an FBI investigation that continued for months after Trump won the presidency.