Social media threats to “shoot up” Salmen High School were pranks, police said, but they caused a major stir at the Slidell public school Tuesday, with police officers on campus throughout the day.

Police learned of the threats early and were at the school when it opened, according to Detective Daniel Seuzeneau, a Police Department spokesman.

The school operated normally, albeit with reduced attendance as a steady stream of parents showed up to take their kids out of school.

By the end of the day, about 500 of the school’s approximately 900 students had left early, according to Meredith Mendez, a school system spokeswoman.

Salmen Principal Brennan McCurley sent a recorded message to parents and a letter was sent home with each student, Mendez said. Another automated call was set to go out Tuesday night, she said.

“We are going to encourage parents to send their kids to school tomorrow and tell them that we feel it’s safe,” Mendez said.

Slidell police were still investigating but said there were never any signs that the threats were legitimate. Police Chief Randy Smith vowed that those responsible for the threats would be arrested.

“I can assure you this alleged prank was not funny, and the person(s) responsible will be charged with a serious felony crime,” he said in a statement.

Those involved could be charged with terrorizing, Seuzeneau said. Detectives requested information from Twitter on the account or accounts where the threats were made, he said. In the meantime, he encouraged anyone with knowledge of the threats to contact the Slidell Police Department through Crimestoppers at (504) 822-1111, through the department’s mobile app, by phone at (985) 643-3131 or on Facebook.

Seuzeneau couldn’t confirm the exact Twitter account on which the threats were made, but screen shots of tweets identifying the high school and promising to “kill thousands” were circulating among parents and others in Slidell. The tweets also threatened to kill particular students and their friends “at salmen,” according to the images.

The fact the threats were made on social media played both positive and negative roles in their detection and police response, officials said. School officials spent part of the day refuting erroneous reports of lockdowns and school evacuations, while the police first became aware of the situation through shared posts, Seuzeneau said.

Follow Faimon A. Roberts III on Twitter, @faimon. Follow Sara Pagones on Twitter, @spagonesadvocat.