The Orlando shooter's father said terrorists planted the idea of carrying out a mass shooting in his son's head -- and he didn't even know it was happening.

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“My son was killed by terrorists,” Seddique Mateen said Wednesday morning from his home in Port St. Lucie, Florida. “Fifty people were killed by terrorism. This was a terrorist idea. (My son) learned from them.”

Mateen said he didn’t notice any signs or changes in Omar Mateen, the 29-year-old who opened fire on a crowd at Orlando’s Pulse nightclub early Sunday, slaying 49 people before he was killed in a police shootout. Seddique Mateen said he doesn’t know how his son connected with the Islamic State, to which Omar Mateen pledged his allegiance in a 911 call before the shooting.

“I don’t want to torture myself (about signs I may have missed). I did my best,” he said. This tragedy should shift the country’s focus to “annihilating” terrorism, he added.

“(Omar Mateen) was used by (terrorists),” he said. “He learned from them.”

Although the younger Mateen had been interviewed twice by the FBI, including once for inflammatory comments to co-workers about possible terrorist ties, the family didn't consider this a sign or notice any changes in his behavior following the incident, Seddique Mateen said.

“If the problem goes away at your job, you’d be happy, right? He was very appreciative of law enforcement (after the FBI interview),” he said.

Growing up, Omar Mateen was active, a little restless in class and social, his father said. He adhered to religious obligations, like praying and reading the Quran, the holy book in Islam. Until he moved out at the age of 18, Omar Mateen seemingly never drank or experimented with drugs, his father said.

Seddique Mateen was unsure what his son did “behind closed doors” after moving out, he said.

Mateen said parents should learn from his experience and have a “closer relationship” with their children.

“Based on what I thought, (our relationship) was close,” he said of his son. “But he fooled me.”