John Mindermann is part of an unusual fraternity. A former agent with the Federal Bureau of Investigation, now 80 and retired in his hometown, San Francisco, he is among the relative handful of law-enforcement officials who have investigated a sitting president of the United States. In June, when it was reported that the former F.B.I. director Robert Mueller would investigate whether President Trump had obstructed the federal inquiry into Russia’s meddling in the 2016 presidential election, I called Mindermann, who told me he was feeling a strong sense of déjà vu.

Mindermann joined the F.B.I. 50 years ago, after a stint with the San Francisco police force, whose corruption he was happy to leave behind. He was soon transferred to the bureau’s Washington field office, housed in the Old Post Office building on Pennsylvania Avenue — the same 19th-century edifice that is now a Trump hotel. On the afternoon of Saturday, June 17, 1972, he was in the shower at home when the phone rang.

An F.B.I. clerk told him that there had been a break-in overnight at the Democratic Party headquarters in the Watergate complex. He was to go to the Metropolitan Police Department headquarters and see the detective on duty. Then, lowering his voice, the clerk confided that the bureau had run a name check on one of the burglars, James McCord. It revealed that McCord had worked at both the F.B.I. and the C.I.A. He would later be identified as the chief of security at the Committee to Re-elect the President, the Nixon campaign operation known as Creep.

Mindermann met the detective, who was wearing a loud sports jacket and smiling widely. The detective strode into the walk-in evidence vault and, wearing latex gloves, produced nearly three dozen crisp new $100 bills, each in a glassine envelope. He fanned them out on a desk, like a magician performing a card trick. They had been seized from one of the burglars. Mindermann noticed the consecutive serial numbers. ‘‘That alone told me that they came from a bank through a person with economic power,’’ Mindermann told me. ‘‘I got this instant cold chill. I thought: This is not an ordinary burglary.’’