FBI investigators collect evidence, including a rifle, where two gunmen were shot dead after their bodies were removed in Garland, Texas, May 4, 2015. REUTERS/Laura Buckman

When Phoenix resident Elton Simpson and his accomplice Nadir Soofi opened fire outside of an art show exhibiting caricatures of the Prophet Muhammad in Garland, Texas, on Sunday, local police had already been tipped off by the Federal Bureau of Investigation that Simpson may pose a danger.

FBI Director James Comey said on Thursday that his agency issued a bulletin to the Garland police department that included a photo of Simpson just hours before the affront, though the agency did not have any evidence to suggest that Simpson was planning an attack. A security guard was injured by the gunmen and both Simpson and Soofi were killed by a police officer shortly after they drove up to the conference center where the event was held and began shooting.

Comey said staff at the FBI’s Dallas field office was aware of the art show, which was billed as a free speech event, and had been on the lookout for potential threats. The show was organized by a group called the American Freedom Defense Initiative which has been described as anti-Islamic by the Southern Poverty Law Center. The FBI warned Garland police about several other possible attendees in addition to Simpson. The agency does not believe that the officer who shot the men had been informed of the potential threat, the Wall Street Journal reports.

Simpson was just one of hundreds of Americans that the FBI monitors who may pose a threat to the nation based on extremist views, NBC reports. “I know there are other Elton Simpsons out there. Many of them we’re covering right now,” Comey said. “But I also know there are Elton Simpsons out there that we have not found and I cannot see.”

Though the Islamic State claimed responsibility for Sunday's attack, Comey did not say whether the FBI had established a link between Simpson and any Islamic militant group. The FBI had prosecuted Simpson in 2011 for lying about his intent to join a Somali-based terrorist organization. Simpson’s case was closed in 2014 after he served three years of probation and once the FBI’s Phoenix field office determined that he was no longer a threat. The agency had reopened his file in March after social media activity suggested Simpson had become interested in the Islamic State group. They alerted local police after Simpson contacted an American in Somalia who encouraged followers to attack the art show, CNN reports.

Simpson’s case as it stands today, as an individual actor who is loosely connected to far-away militant groups through the Internet and social media, represents a new and ambiguous threat for the FBI to monitor. Comey is worried that the Islamic State’s fervent use of social media is particularly effective in moving followers to violence because new reminders are constantly popping up in the form of push notifications on smartphones. He says this constant connectedness stands in stark contrast to the days when followers visited a website in order to receive such a message.