A KGTV news station viewer saw the post in the men-seeking-men section of the Craigslist San Diego personal advertisements. He took a screenshot and sent it to the TV station before the post was flagged and removed.
The post was titled, "We need more Orlando's (sic)," and it was accompanied by a photo of a hand firing a revolver with a bullet coming out of the barrel.
"Orlando was long overdue. Cleanse your community of the filth that gives decent gay men and women a bad name...San Diego you are next...," the post read.
FBI Special Agent Darrell Foxworth was quoted as saying that the bureau's Joint Terrorism Task Force is investigating the Craigslist posting.
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"This is something that's very serious, very concerning to all of us, especially given what's happened recently in Orlando," said Foxworth.
"We are going to use all lawful means available to us, so if we have to go out and get a search warrant, we will do that. If it doesn't require a search warrant, we're going to do whatever it takes to do it in a lawful and legal matter," Foxworth said.
The post comes days after Afghan-origin Omar Mateen carried an assault rifle and a pistol into a gay club in Orlando in the wee hours of Sunday and killed 50 people while injuring 53 others. He was later shot dead by the police.