A chance encounter with a senior FBI official in 1970 grew into a friendship and as the Watergate scandal unfolded the official became the “Deep Throat” source for the stories that helped bring down President Richard Nixon, reporter Bob Woodward wrote in The Washington Post on Thursday.

Woodward said he was a young Navy lieutenant on an errand at the White House when he first met Mark Felt, who was unveiled this week as the instrumental source for the Post’s Watergate stories written by Woodward and Carl Bernstein.

“I asked Felt for his phone number and he gave me the direct line to his office,” Woodward wrote. He said he came to regard the senior FBI official as a friend and a mentor and kept in touch with him. Felt was promoted to the No. 3 position at the FBI in July 1971, about a year before J. Edgar Hoover’s death and the Watergate break-in, and two months before Woodward joined The Washington Post.

Woodward said he turned to Felt after he and Bernstein wrote about the break-in at the offices of the Democratic National Committee at the Watergate complex in Washington.

“This was the moment when a source or friend in the investigative agencies of government is invaluable,” Woodward said. “I called Felt at the FBI … It would be our first talk about Watergate.” Woodward said Felt told him the Watergate burglary case was going to “heat up” for reasons he could not explain and abruptly hung up. But he started providing guidance on the story, Woodward said.

“Felt believed he was protecting the bureau by finding a way, clandestine as it was, to push some of the information from the FBI interviews and files out to the public, to help build public and political pressure to make Nixon and his people answerable. He had nothing but contempt for the Nixon White House and their efforts to manipulate the bureau for political reasons,” Woodward wrote.

Given the complexity of the fast-breaking Watergate story, Woodward said, there was “little tendency or time” to consider the motives of sources. He also noted that Felt was immensely disappointed at being passed over to succeed Hoover.

The clandestine exchanges began when Woodward went to Felt’s home in suburban Virginia one night after failing to reach him by telephone. “He said no more phone calls, no more visits to his home, nothing in the open,” Woodward wrote, saying he was instructed to follow strict counter-surveillance techniques for their meetings at a parking garage in Rosslyn, Virginia, across the Potomac River from the U.S. capital.

He and Felt worked out a notification system for when either of them wanted a meeting, Woodward said. He said he signaled Felt using a small red flag in an empty flowerpot on his apartment balcony. Felt would put a signal in Woodward’s copy of The New York Times delivered outside his apartment.

“How this was done, I never knew,” Woodward wrote. “Page 20 would be circled, and the hands of a clock in the lower part of the page would be drawn to indicate the time of the meeting that night.”

Woodward said he did not know at the time that in Felt’s earliest days at the FBI, during World War II, Felt had learned a lot about German spying and after the war spent time keeping suspected Soviet agents under surveillance.