Police and FBI on Monday searched the Arizona apartment of one of two gunmen shot dead on Sunday after they allegedly opened fire with assault rifles outside a Texas exhibit of caricatures of the Prophet Mohammad.
Police and federal agents had planned security for months ahead of the event in the Dallas suburb of Garland, which was organized by American Freedom Defense Initiative (AFDI), a free-speech organization that is also described as a hate group, and that paid $10,000 for extra protection.
The shooters, who injured a security guard before they were shot dead by a police officer using his duty pistol, wore protective gear and carried extra ammunition in their car, Garland police spokesman Joe Harn said. No bomb was found in their vehicle.
Two law enforcement officials, who asked not to be named, said one of the dead shooters was Elton Simpson of Arizona, who had been monitored by the FBI since 2006 and had been convicted for lying to FBI agents over his desire to join violent jihad in Somalia.
An FBI official said agents were at the apartment where Simpson lived in Arizona.
The incident in the Dallas suburb was an echo of past attacks or threats in other Western countries against art depicting the Prophet Mohammad. In January, gunmen killed 12 people in the Paris offices of French satirical magazine Charlie Hebdo in what it said was revenge for its cartoons.
Police in Garland said a bomb squad, the Federal Bureau of Investigation, a SWAT team and the US Bureau of Alcohol, Tobacco, Firearms and Explosives had been involved in preparations for security around the controversial exhibit and contest at the Curtis Culwell Center.
"Obviously they were there to shoot people," Harn said at a news conference.
Referring to the police officer who killed the attackers in the parking lot as the event was ending, Harn said, "He did a good job."
Some 200 attendees inside the arena were not aware of the attack outside.
At the sprawling, well-kept apartment complex in north-central Phoenix, police tape blocked off a portion of a brown, two-story building.
Phoenix FBI spokesman Perryn Collier confirmed agents were at the Autumn Ridge Apartments in connection with the shooting in Texas. It was not immediately clear if both gunmen had lived in the complex, or whether they shared an apartment.
Wanted to join jihad
Simpson, believed to be one of the dead shooters according to law enforcement sources, was convicted in 2010 of lying to FBI officials over discussions he had with an informant about his desire to travel to Somalia to engage in violent jihad.
According to court records, Simpson waived his right to a jury trial and was tried before Judge Mary Murguia, who sentenced him to three years probation and ordered him to pay $600 in fines and penalties.
The court documents say federal authorities began monitoring Simpson in 2006 because he was associated with an individual the FBI believed was trying to set up a terrorist cell in Arizona.
At one point, according to the documents, the FBI tried "unsuccessfully" to put Simpson on a US government no-fly list.
Judge Murguia found Simpson guilty of making a false statement but said there was insufficient evidence to conclude the false statement involved international terrorism.
ABC News said officials believed Simpson sent out tweets ahead of the attack, with the last one using the hashtag #texasattack.
US authorities were investigating possible links between the gunmen and international terrorist groups, a government source said.
President Barack Obama asked his advisers to keep him up to date on the investigation, White House spokesperson Josh Earnest told reporters traveling with the president on Air Force One.
Free speech or hate
AFDI's event in Garland, Texas was called "Muhammad Art Exhibit and Contest" and offered a $10,000 prize for the best artwork or cartoon depicting the Prophet.
Depictions of the Prophet Mohammad are considered offensive in Islam, and Western art that portrays the Prophet has angered Muslims and provoked threats and attacks from extremists.
Ahead of the event, Dallas Muslims had circulated messages on social media urging each other to ignore the event and not give the organizer any attention.
The event featured speakers including Geert Wilders, a polarizing Dutch politician and anti-Islamic campaigner who is on an al Qaeda hit list.
The AFDI, described by the Southern Poverty Law Center as a hate group, has among other activities sponsored anti-Islamic advertising campaigns in transit systems across the country.
Pamela Geller, AFDI president, defended the event on Monday, telling CNN: "In a pluralistic society you have offensive speech, you have ideas, you have an exchange of ideas. You don't shut down a discussion because 'I'm offended.'"
In Garland on Monday, police crime scene investigators searched the parking lot for evidence. A police helicopter flew overhead and the area was cordoned off by police.
Police said that people who attended the event had been taken in a bus to a hotel, and would later be allowed to go back to get their cars, which were left at the arena.
Texas: 2 suspected of carrying bombs killed at Prophet cartoon event
No joke this: A timeline of Prophet cartoons controversy