A St. Paul middle school postponed a daytime dance slated for Wednesday after staff learned of a threatening social media post.

In letters to Murray Middle School families, Principal Jamin McKenzie said that a recent Snapchat message included “threatening language about the dance.” The school resource officer and district security staff could not determine who posted the message.

McKenzie said staff decided Tuesday night to postpone the dance “to have a normal school day to promote a sense of calmness and safety.”

Additional security guards were posted at the school Wednesday.

No one has been arrested and the St. Paul police department is investigating, spokesman Sgt. Mike Ernster said Wednesday afternoon.