Two boys were questioned by police after swastikas were daubed on a school wall.
The Nazi graffiti was found at a school in Headington, Oxford, and appeared to be inspired by the recent terror attack in Christchurch, New Zealand.
A 14-year-old boy and a 15-year-old boy were questioned under caution, and the 14-year-old was also interviewed about a similar incident in November.
They have been referred to the Youth Justice Service, police said.
The graffiti on the wall in Cheney Lane, also said "Sub to PewDiePie" - a reference to the popular YouTube star, whose name was also mentioned by the Christchurch attacker.
Supt Joe Kidman, of Thames Valley Police, said: "I deplore the actions of the person or persons responsible for this graffiti and there are no excuses for those who carry out hate crimes.
"Our thoughts remain with the people of New Zealand and we stand together with them and all our communities at this difficult time."
In November, swastika graffiti appeared on and around Headington Quarry Pavilion but was quickly replaced by posters saying "stand up to racism".
Labour city councillor Shaista Aziz spotted the swastikas while dropping off her nieces at school.
She tweeted: "Absolutely disgusted to see Nazi signs sprayed on a wall and building in Oxford this morning... this is how hate is normalised."