FBI agents have searched an apartment in Phoenix as part of an investigation into a shooting outside a Dallas venue hosting a contest for Prophet Muhammad cartoons.
A police officer shot and killed two gunmen who opened fire outside the event last night. A security officer was wounded in the shooting.
Garland police officer Joe Harn says the men had used assault rifles and that one officer had fatally shot both gunmen.
Mr Harn also said investigators searched the men's car and detonated several suspicious items, but no bombs were found in the vehicle.
"We were able to stop those men before they were able to penetrate the area and shoot anyone else," Mr Harn said.
The cartoon event yesterday featured speeches by American Freedom Defence Initiative president Pamela Geller and Geert Wilders, a Dutch lawmaker known for his outspoken criticism of Islam.
Mr Wilders , who has advocated closing Dutch doors to migrants from the Islamic world for a decade, has lived under round-the-clock police protection since 2004.
The FBI said the Phoenix residence was being searched for indications of what prompted the attack and spokeswoman Katherine Chaumont said no other locations in Phoenix are being investigated.
Agents could be seen also searching a white Chevy minivan. They took what appeared to be plastic bottles out of the vehicle.
A federal law enforcement official has identified one of the suspects in the shooting as Elton Simpson and claimed investigators were searching his property in connection with the case.
Court documents show a man by the name of Elton Simpson was convicted in 2011 in federal court in Phoenix of making a false statement by lying to an FBI agent in January 2010 about whether he had discussed travelling to Somalia.
According to the documents, Simpson had discussed with an FBI informant a desire to travel to Somalia but denied to an FBI agent that he had had any such discussions.
According to trial testimony, Simpson is an American Muslim who became the subject of a criminal investigation in 2006 because of his association "with an individual whom the FBI believed was attempting to set up a terrorist cell in Arizona", US District Judge Mary H Murgia said in her order convicting Simpson.
Prosecutors alleged that the false statement involved terrorism but Ms Murgia's order said prosecutors had not proved that part of the allegation. Another federal judge later sentenced Simpson to three years of probation.
A resident of the Phoenix apartment complex, Douglas Hayes, said he saw police cars flood the complex last night and that SWAT team members walked throughout the complex.
The bodies of the men could still be seen on the ground near the car today before they were later covered up. Investigators are still processing the crime scene, Mr Harn said.
The contest, hosted by the New York-based American Freedom Defence Initiative, offered a 10,000 US dollar (£6,610) award for the best cartoon depicting the Prophet Muhammad.
According to mainstream Islamic tradition, any physical depiction of the Prophet Muhammad - even a respectful one - is considered blasphemous. Drawings similar to those featured at the Texas event have sparked violence around the world.
White House spokesman Josh Earnest said President Barack Obama was informed about the shooting. He said the president believes there is no form of expression that would justify an act of violence.
The wounded security officer was shot in the lower leg, Mr Harn said. He was treated and released from hospital.