Swastikas, anti-Semitic messages and gang graffiti were found scrawled at Syosset High School early Sunday morning.
A security guard found the graffiti on walls, doors and windows in the rear of school, which is located on Southwoods Road.
Police say "MS-13" and other profanities had been scrawled in various colors of spray paint.
They say the incident occurred sometime between the hours of 2 a.m. and 4 a.m. Sunday.
Detectives are asking anyone with information to call Nassau Crime Stoppers at 800-244-TIPS.
Syosset Board of Education Trustee Joshua Lafazan says he will call for stepped-up security at all Syosset school buildings at the next board meeting.
County Legislator Arnold Drucker says he's asked law enforcement officials authorities to boost patrols in the area, and he condemned the hateful writings as "sickening and repulsive."