He was investigated twice, and the FBI found nothing and dropped it. 2013 and 2014. He was cleared.
You're kinda ignorant on this issue, aren't you?
How is a person "stopped" when the FBI does its job and discovers nothing? Explain.
Just because he was "cleared" and it dropped doesn't mean they forget otherwise his name wouldn't have kept coming up....
"In May 2013, co-workers at the courthouse reported Mateen had claimed he had family connections to Al Qaeda and had relatives who were in Hezbollah, two groups that are opposed to ISIS, FBI Director James Comey said. He also told colleagues that he hoped law enforcement would raid his home and assault his wife and child "so he could martyr himself," Comey said.
The FBI investigated, using informants, recording conversations with Mateen, and interviewing him twice, Comey said. Mateen admitted making the statements to his colleagues, but said it was because he was angry they were teasing him for being Muslim. The probe ended in May 2014.
Two months later, in July 2014, Mateen's name came up during an investigation of a Fort Pierce man, Moner Mohammad Abu-Salha, a Palestinian-American who carried out a suicide attack in Syria on behalf of the Al-Nusra front, according to The Associated Press. Comey confirmed that account, but didn't identify the bomber, saying the two men knew each other "casually" from attending the same mosque. An investigation determined there were no ties of any consequence, Comey said.
