For one man, the Robert A. Durst chronicle is not only about unsolved deaths from the distant past.

His younger brother Douglas, 70, believes that Robert, 71, has contemplated killing him for years, and even continued to stalk him last month, as HBO was running “The Jinx,” a documentary series that chronicled the violent deaths or disappearance of three people close to Robert.

“We had information that Bob was five or 10 minutes away from my house in Florida,” Mr. Durst said, in his first interview since the documentary was broadcast.

On the evening of Feb. 22, relatives staying at Douglas Durst’s home in Palm Beach learned that Robert was in Riviera Beach, less than nine miles north, according to Douglas. That night, the third chapter in the documentary was being shown.

The Dursts, members of a New York City real estate dynasty, do not know if he got as far as the Palm Beach house, Douglas said.