Police search the home of a suspected gunman who was shot dead in Texas after attacking a controversial event showing caricatures of the prophet Mohammed, organised by an anti-Islamic group.
An FBI spokesman said agents were examining an apartment in Phoenix, Arizona for evidence.
US media reports that one suspect has been identified as Elton Simpson, who had previously been investigated on suspicion of terrorism offences.
There are similarities with past attacks or threats in other western countries where cartoons have depicted the prophet Mohammed.
In January, gunmen killed 12 people in the Paris offices of French satirical magazine Charlie Hebdo for publishing cartoons depicting Muhammad.
The exhibition, at the Curtis Culwell Center in Garland, close to Dallas, Texas was described by organisers as a free-speech event. The shooting happened at about 7pm in a carpark outside the conference centre.
The gunmen were shot close to the car they had arrived in. The two suspects drove up to the building as the event was ending, and opened fire with automatic rifles at an unarmed security officer, striking him in the leg, police and city officials said.
Garland police officers who were assisting with security returned fire, killing both suspects. One of the suspects, after being initially wounded by police gunfire, was seen reaching for a backpack and was shot again and killed, Garland Mayor Douglas Athas later told CNN. The security officer was treated at a local hospital and later released, Harn said. No one else was injured.
Shortly before midnight police alerted media that a strong electronic pulse would be activated near the scene, presumably as part of the bomb squad’s work, and a loud boom was heard moments later, though police did not comment further on what was done.
The event was organised by Pamela Geller, a campaigner who is virulently anti-Islam and who co-founded the co-founded the group Stop Islamization of America.
In 2013 she was banned from entering the UK, together with the other co-founder of the group, Robert Spencer.
The pair had been due due to attend a march planned by the far-right EDL to mark Armed Forces Day on 29 June, ending in Woolwich, south east London, where soldier Lee Rigby was murdered in a terrorist attack. At the time, the anti-fascist campaign group Hope Not Hate described them as some of the most extreme anti-Muslim activists in the world and said they had nothing to contribute to life in the UK.
The event in Dallas, described as the “Muhammad Art Exhibit and Contest”, had been offering a $10,000 prize for the best artwork or cartoon depicting the prophet, as well as a $2,500 “People’s Choice Award.” Depictions of the Prophet are viewed as offensive in Islam, and western satirical cartoons that portray him have sometimes angered Muslims and provoked threats and attacks from radicals.
The mayor of Garland, Douglas Athas, said the city had permitted the event even though officials knew its inflammatory theme could provoke an attack.
“There was concern, which is why we had heightened security in the area, but we all swear to uphold the constitution: free speech, free assembly and in this case perhaps, free religion,” Athas said.
“So in this case they were free to use the building.”
He said the school district that owns the building had posted extra security officers at the venue, and the city of Garland also had a number of security and Swat (special weapons and tactics) teams in the area.