... Authorities named Nikolas Cruz — a 19-year-old former student of Marjory Stoneman Douglas High School — as the suspect in custody for the crime. Almost immediately after his name was made public, social media users, apparently inspired by nothing more than a Spanish-sounding surname, began spreading an unfounded claim that he is an undocumented DACA recipient ...

However Cruz was adopted at birth by a couple with that surname and is not undocumented ...

https://www.snopes.com/florida-high-school-shooter-dreamer/

... Mr Cruz may have left warning signs on social media. A person with his name wrote a comment last year under a YouTube video that read "I'm going to be a professional school shooter".

The man who posted the video, Ben Bennight, was alarmed and contacted the FBI, according to a video he posted online.

"No other information was included with that comment which would indicate a time location or the true identity of the person who made the comment," FBI Special Agent in Charge Robert Lasky said.

Investigators were unable to find the commenter ...

http://www.abc.net.au/news/2018-02-16/florida-shooting-fbi-warned-about-alleged-gunman-nikolas-cruz/9453156

Yeah, you suspect something might happen. Where, when, to whom and by whom ... Total mysteries.

Even "with high confidence AQ will attack the US" leaves open whether it'll be in Africa or Kansas, mid-August or mid-December, by plane, train, or assault by individuals in a dozen places at once, or maybe car bomb, against infrastructure or against people. Even knowing "by plane" would have meant imposing draconian regulations on travelers, many of whom hated Bush II already and would see no useful purpose in it. Then, if it was successful in blocking it because the terrorist called it off, when would we know to stop it. And if we contacted the, gee, a bunch of students who overstayed their visas and were going to use box cutters. It would be deprecated to no end. In other words, stopping 9/11 would have been a problem, might not have worked, and if it was stopped the measures used would have been a problem.

by MICHAEL KOSNAR and DANIEL ARKIN

... A motive in the shooting remains unclear, but students who encountered Cruz said he talked a lot about having fun with firearms, so much so that some joked that if anyone were to attack the school, it would probably be him.

Cruz "threatened to bring the guns to school multiple times," senior Eddie Bonilla said. Other students, he said, "threw jokes around that he'd be the one to shoot up the school" ...

https://www.nbcnews.com/news/us-news/florida-shooting-fbi-was-alerted-about-threat-posted-youtube-n848316

Tina Bellon, Jan Wolfe

(Reuters) - A few U.S. states have laws allowing police and family members to obtain orders barring people suspected of being a threat from possessing guns, but Florida does not. Some gun control proponents and legal experts said Wednesday’s school shooting might have been thwarted if it had ...

A federal GVRO law was proposed last year, and its supporters in Congress on Thursday urged its passage ...

... A judge can issue an initial restraining order and confiscation of firearms without a hearing.

The NRA did not respond to a request for comment on Thursday.

https://uk.reuters.com/article/uk-florida-shooting-law-analysis/gun-restraining-order-might-have-thwarted-florida-shooting-experts-idUKKCN1FZ313

... On Thursday, Mr Cruz confessed to investigators that he carried out the shooting, adding that he had a backpack full of ammunition ...

... maths teacher Jim Gard told the Miami Herald newspaper that school authorities had emailed teachers about Mr Cruz's behaviour ...

"There were problems with him last year threatening students, and I guess he was asked to leave campus" ...

... Former schoolmate Joshua Charo said Mr Cruz had been found with bullets in his backpack ...

http://www.bbc.com/news/world-us-canada-43071710