Robert F Kennedy Jr revealed his father spent a year trying to come to grips with the death of his b [PA]

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Mr Kennedy and his sister, Rory, were interviewed in front of an audience in Dallas as a year of observances begins for the 50th anniversary of the president's death.

Their uncle was killed on November 22, 1963. Five years later, their father was assassinated in a Los Angeles hotel during his primary victory celebration.

Robert Kennedy said his father spent a year trying to come to grips with his brother's death. He said he read writings by Greek philosophers, Catholic scholars, poets and Henry David Thoreau "trying to figure out kind of the existential implications of why a just God would allow injustice to happen of the magnitude he was seeing".

He said his father believed the Warren Commission, which concluded Lee Harvey Oswald acted alone in killing the president, was a "shoddy piece of workmanship". He said that he, too, questioned the report.

"The evidence at this point is convincing that it wasn't a lone gunman," he said, but he did not say what he believed may have happened.

The lawyer and well-known environmentalist also told the audience light-hearted stories about memories of his uncle.

As a young child with an interest in the environment, he said, he made an appointment with his uncle to speak with him in the Oval Office about pollution.

He had even caught a salamander to present to the president, which unfortunately died before the meeting.

"He kept saying to me, 'It doesn't look well'," Robert Kennedy recalled.