1. The F.B.I. opened an inquiry into whether President Trump was secretly working with Russia after he fired James Comey as F.B.I. director, according to former law enforcement officials and others familiar with the investigation.

The inquiry carried explosive implications. Counterintelligence investigators had to consider whether the president’s own actions constituted a possible threat to national security.

The special counsel, Robert Mueller, took over the inquiry when he was appointed days after F.B.I. officials opened it. The investigation also had a criminal aspect, which has long been publicly known: whether Mr. Trump’s firing of Mr. Comey constituted obstruction of justice.