Phoenix police and the FBI gathered intelligence and monitored social media, but they say they were more concerned about people who are staying under the radar than they are about organizers and worshippers.
This is the same mosque that Elton Simpson and Nadir Soofi attended. They were shot and killed during their attack at a Prophet Muhammed cartoon drawing contest.
Friday's rally is in response to that terrorist incident in Texas, but it's not just worshippers who are on edge. Neighbors are also nervous about what may happen.
"I mean it's pretty scary because it's putting my kids in danger. It's just a religion. Everybody is allowed to practice whatever religion they like. I'm just guessing I'm going to take off with my kids during the time they come," said Adriana Viguiero, a neighbor.
Friday is Jummah, the Muslim day of communal prayer.
This is mosque also is one of two in the valley that have received threatening letters in the mail. The FBI is still investigating.