A 15-year-old boy was arrested for threatening copycat killings on Instagram after the Florida high school shooting

AP

A sheriff's vehicle is seen outside Marjory Stoneman Douglas High School after the mass shooting.

• An unnamed teenage boy was arrested for threats he made after America's latest mass shooting.
• Officials say he threatened to kill people and listed a number of schools.
• They were all in the same district as Marjory Stoneman Douglas High, where 17 people were killed on Wednesday.
• Detectives said he told them the post was a joke to scare his cousin.
• If prosecuted and convicted he could spend five years in jail.

A fifteen-year-old boy has been arrested in Broward County, Florida, after claiming online that he was planning a follow-up attack after the mass shooting in the US state last week.

Sheriff's deputies took the boy into custody after his post, made on Instagram, was reported to them. It follows the massacre of 17 people at Marjory Stoneman Douglas High School on Wednesday.

Nikolas Cruz, a 19-year-old former student, has been charged with the killings and faces the death penalty if convicted.

The sheriff's office said the threat was posted less than 48 hours after the attack. They said that the boy used to post to say "he was going to kill people" and "listed various schools" that could be his target.

They said that when detectives arrested the boy, he "appeared to be remorseful and claimed his post was a joke aimed at scaring his cousin." They did not publish his name.

He is facing a charge of "sending a written threat to kill," a third-degree felony in Florida. Such crimes have a maximum punishment of five years in prison.

Susan Stocker/Reuters

Nikolas Cruz at a preliminary court hearing after the shooting.

Cruz was arrested in the hours after the shooting and remains in police custody. Although he is still formally a suspect, his guilt could be established quickly. 

According to the sheriff's office, Cruz confessed to the shooting. Cruz's defense lawyer says that he will "never see the light of day again," and that he will probably plead guilty to the murder charges if he is spared the death penalty.

Read all Business Insider's coverage of the Florida shooting and its aftermath here.