JACKSON, Miss. - A Mississippi high school senior was jailed without bail Friday after the FBI charged him with threatening a school shooting over social media, but his defence lawyer says officials are misreading the student's dark humour.

After a hearing in federal court in Oxford, U.S. Magistrate Judge Roy Percy ordered 19-year-old Nathan Caleb Brown of Southaven held in the Lafayette County jail, finding him too dangerous to release. Court papers show officials confiscated three handguns, two knives and ammunition from Brown's home.

The FBI arrested Brown on Thursday at DeSoto Central High School in Southaven. Court papers show Brown admitted the postings in questioning at the school.

"Brown stated the whole basis for these tweets was to be funny online," wrote Special Agent Charles Boling in a sworn criminal charge filed Thursday. Boling added Brown was asked "how he thought normal people would receive the posts, and Brown responded that he could see where it would be concerning for people outside his social group."

The charge of transmitting a threat across state lines carries a prison sentence of up to five years in federal prison.

Defence lawyer Steve Farese said Brown's parents testified at the Friday detention hearing has never been in trouble, has no violent tendencies and has never owned or fired a gun. Farese said it's "undisputed" that the guns, including one that was loaded, belong to Brown's mother and were found in her bedroom.

Farese said he Brown's actions don't justify him being charged with a federal crime, saying Brown and other teens communicate things electronically that would have never been circulated widely in previous generations.

"No longer can you sit at home and write in your personal journal your fantasies," Farese said. "You cannot put tone in an email or a text. You cannot understand the context of a dark joke."

The FBI, though, disagrees. Boling wrote that two different people outside Mississippi had reported Brown's tweets to the FBI "as potential threats of violence."

The charge cites a number of tweets that Brown made between February and April, including one saying he wanted to "make a Columbine v2," or version two of the 1999 shooting at the Colorado high school; one telling another user "I can't believe you shot up a school without me" on the day of a shooting at Great Falls High School in Maryland; and a tweet Monday threating a shooting at Snapchat headquarters after the service locked Brown's account.

On April 1, court papers say Brown responded to a user who sent some of the tweets to the FBI saying "They knew about Nikolas Cruz too," referencing the person accused in Feb. 14 shooting at a Parkland, Florida, high school that killed 17. The Cruz tweet zeroes in on criticism of the FBI for ignoring tips warning about Cruz.

"I understand what they did and I understand why they did it," Farese said of the FBI.

Officials began investigating after a March 19 tweet was reported, eventually tracked an internet address to a Southaven home, and observed Brown leave home and drive to DeSoto Central on Wednesday.

Boling said a brief search of Brown's phone after his arrest "revealed numerous searches involving serial killers and numerous photos of Columbine and other school shootings."

The case will be referred to a grand jury and Farese said he hoped at some point to meet with prosecutors to discuss "the totality of the circumstances."

Even if he's not convicted, Brown could face consequences up to expulsion for year at the high school from which he had been scheduled to graduate from in May, according to DeSoto County school discipline policies. Spokeswoman Katherine Nelson issued a statement acknowledging the arrest, but declined further comment.

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