Adnan Agca said that his brother told him Wednesday in Istanbul's Kartal prison that he hoped the pope lived a bit longer.
"I know that he is in mourning. I feel that he is in deep sorrow over the death of the pope, who was like a brother to him," Adnan Agca told The Associated Press by telephone.
"I feel strongly that my brother is in deep grief. We're all very sad," Adnan Agca said. "He was a great man who contributed a lot to world peace."
The pope forgave Mehmet Ali Agca and met him in his Italian prison cell in 1983. Agca was extradited to Turkey in 2000 after almost 20 years behind bars in Italy for the shooting.
"They had declared brotherhood when the Pope visited him in prison," Adnan Agca said of the 1983 meeting. "He was Agca's brother, would not you be sad if you had lost your brother?"
Adnan Agca said the pope has received relatives of the gunman several times over the past years, meeting Agca's mother, Muzeyyen, in 1987 and Adnan Agca in 1997.
Adnan Agca said he was considering attending the pope's funeral.
"We may go to Italy if possible in a few days," Adnan Agca said.
Agca, who is serving a 17-year prison sentence near Istanbul for earlier crimes in Turkey, has given conflicting reasons for his 1981 assassination attempt against the pope in St. Peter's Square.
Suspicions that the Turk acted on behalf of the former Soviet bloc, which feared that the Polish-born pope would help trigger anti-communist revolts, linger despite denials by former Soviet President Mikhail Gorbachev.
The Turk initially claimed he was acting alone in the attack. Later he said he was trained by Bulgarian and Czech experts and blamed the KGB for the attempted assassination.
During his trial in Italy, however, Agca said part of his testimony had been lies, and Italian courts ruled there was insufficient evidence to support claims of Soviet involvement.
Italian newspapers reported earlier this month that documents discovered recently in the archive of the Stasi, the secret police of former East Germany, appear to link Bulgaria to the attack.
Bulgarian officials denied the allegations Friday.
In the pope's newest book, released in February 2005, the pontiff said Agca was a "professional assassin" who carried out the shooting for somebody else.
John Paul did not directly say who he thought was responsible, but called it "one of the last convulsions of the 20th Century ideologies of force," which he said included communism.
During a 2002 trip to Bulgaria, the pope dismissed any Bulgarian connection to the attack.
John Paul long had said he believed the hand of the Virgin Mary deflected Agca's bullet. In turn, Agca has sometimes suggested he was part of God's plan — a claim dismissed by Vatican officials.
In 2000, Agca was extradited to Turkey where he is now serving a 10-year prison sentence for the 1979 murder of a prominent Turkish newspaper editor and an additional seven years for commandeering a taxi and robbing an Istanbul soda factory.
Agca's attorneys claim he could be released from jail as early as 2005 because of recent changes to Turkish law, although it was unclear if authorities would agree to free him.