Seventh Precinct detectives reported the social media threat was made against Seaford High School, located on Seamans Neck Road, on Snapchat, police said.
"At this time the threat is not believed to be credible," police said in a 9 a.m. statement.
They went on to say they believe that all area schools are "deemed safe for staff and students to return."
Nassau County Police Department and Seaford School District said they will continue to work together to investigate the threat.
Anyone with information about this incident or any other threats to Seaford schools is asked to call the Seventh Precinct at (516) 573-6700. If immediate assistance is required, please call 911.
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