According to the Simi Valley Police Department, the first people to arrive before 7 a.m. at Justin Elementary School at 2245 North Justin Avenue found a large amount of threatening graffiti. Police declined to elaborate on the markings.
Officials said the graffiti was immediately painted over. Parents were greeted with a heavy police presence.
"We all agreed that the safest step to take would be to put the students in a lock-in situation, which simply means the students are restricted to their classroom environment and they're escorted to the restrooms until their parents can pick them up," said Lt. Stephanie Shannon of the SVPD.
Mary Eklund, who runs a nearby child care center, said her husband saw the graffiti when walking a student to school in the morning.
"When my husband walked her to school, he noticed swastikas painted on the classroom doors," Eklund said.
Fourth-grader Jordan Hayes said he knew what the symbol meant.
"The Nazi sign," the boy said. "It was a very bad sign. My dad told me about it and it just made me angry."
The school was set to resume its regular schedule Tuesday.