The informant made contact in the summer of 2016 with Trump campaign advisers who were already under scrutiny for ties to Russia.
Mr. Trump has described the investigation as a politically motivated effort to undermine his presidency. But time and again, agents took steps that ultimately benefited him.
The aide said he remembered getting an email about Russian dirt on Hillary Clinton well before compromising Democratic emails were released. No one can find the message he mentioned.
The lawsuit accuses Mr. Trump’s associates of illegally working with Russian intelligence agents to interfere with the outcome of the 2016 election.