A neighbor of one of the teenage killers who went on a deadly shooting rampage at a San Diego mosque described a chilling encounter with the suspect.

Jennifer Du, who lives across the street from Cain Clark, recalled seeing the 17-year-old on Sunday — a day before he and 18-year-old Caleb Vazquez opened fire at the Islamic Center of San Diego, killing three before turning the gun on themselves.

“It’s been unreal. I saw him literally the day before. He just stood there,” Du, 30, told The California Post.

“I didn’t know if he was watching me or something, but he just got food delivery and went back in.”

Clark’s next door neighbor described his family as normal, noting that the teen practiced martial arts and that she saw him in a karate outfit.

“As far as we knew he was very nice. They were a nice family,” said the neighbor, Marne, 85, who declined to provide her last name.

“We were just flabbergasted,” she added of finding out about the attack. “We thought, ‘Oh my god,’ we couldn’t believe it. Just flabbergasted.”

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Clark and Vazquez gunned down three people on Monday, including security guard and father of eight Amin Abdullah, who police say prevented more deaths. Two others members of the mosque, Nader Awad and Mansoor Kazziha, were named as victims of the shooting.

At least one of the suspects took a weapon from his parents’ home and left a suicide note that talked about racial pride, a law enforcement source told The Post.

The shooters co-authored a manifesto, featuring the same Nazi imagery Clark donned in a livestream video of the horrific attack.

After the shooting, the livestream showed Clark shoot Vazquez twice in the head before turning the gun on himself.

Anti-Islamic writings were also found in the suspects’ vehicle, according to a law enforcement source.

The shooters’ motivations are unknown, though it appears to be a crime aimed at the Muslim faith.

Elizabeth Aderhold, who lives across the street from the Islamic Center, said that security beefed up after the Israel-Gaza conflict broke out, and for a while, city cops would come to provide security.

“It’s a pretty safe neighborhood. We never had a problem before,” Aderhold said. “The craziest thing that happened is a garbage truck fire.”