GARLAND, Texas — Federal agents searched an apartment in Phoenix in an investigation into a shooting outside a suburban Dallas venue hosting a provocative contest for Prophet Muhammad cartoons, the FBI confirmed Monday.

Police officers shot and killed two gunmen who opened fire on a security officer outside the Curtis Culwell Center in Garland on Sunday night. The security officer was wounded in the shooting.

The FBI says the Phoenix residence is being searched for indications of what prompted the attack, and FBI spokeswoman Katherine Chaumont said no other locations in Phoenix are being investigated.

Agents could be seen also searching a white Chevy minivan at the Autumn Ridge Apartments complex. They took what appeared to be plastic bottles out of the vehicle.

The apartment is on the first floor of a two-story building. The area around the building is sealed off but residents walked about and stood on their balconies watching law enforcement.

ABC News, citing a senior FBI official, reported that one of the gunmen, a resident of the Phoenix apartment, was known to authorities and has been the subject of a terror investigation. That man was convicted five years ago of lying to federal agents about plans to travel to Africa in an apparent attempt to join a terror group there, ABC reported.

Garland police spokesman Joe Harn said it was not immediately clear whether the shooting was connected to the contest hosted by the New York-based American Freedom Defense Initiative that would award $10,000 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.

Harn said Sunday that a bomb squad was searching the gunmen’s vehicle for explosives. The vehicle was heavily damaged Monday morning, indicating that authorities detonated the car overnight as a precaution.

A resident of the apartment complex, Douglas Hayes, said he saw police cars flood the complex Sunday night and saw SWAT team members walking throughout the complex. Hayes said early Monday morning he heard a loud noise that turned out to be law enforcement personnel breaking into a parked white minivan. Hayes says all the windows were broken, leaving glass scattered about the vehicle.

The bodies of the men could still be seen on the ground near the car Monday before they were later covered with a tarp. Investigators are still processing the crime scene, Harn said.

The wounded security officer, who was unarmed, worked for the Garland Independent School District which owns the center, Harn said. He was treated and released from a hospital.

Harn said the district hires security for events at its facilities, but noted that additional security was hired for Sunday’s event. The sponsoring group has said it paid $10,000 for off-duty police officers and other private security.

Harn said the city had not received any credible threats before the shooting.

Texas Gov. Greg Abbott said state officials are investigating, and Dallas FBI spokeswoman Katherine Chaumont said that agency is providing investigative and bomb technician assistance.

The event featured speeches by American Freedom Defense Initiative president Pamela Geller and Geert Wilders, a Dutch lawmaker known for his outspoken criticism of Islam. Wilders received several standing ovations from the crowd and left immediately after his speech.

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.

After the shooting, authorities escorted about 75 contest attendees to another room in the conference center, where a woman held up an American flag, and the crowd sang “God Bless America.”

The group was then taken to a separate location, where they were held for about two hours where they were questioned by FBI agents then released.

Johnny Roby of Oklahoma City, Oklahoma, told the Associated Press he was outside the building when he heard about 20 shots that appeared to come from a passing car.

Roby said he then heard two single shots. He said he heard officers yell that they had the car before he was sent inside the building.

Geller told the AP before Sunday’s event that she planned the contest to make a stand for free speech in response to outcries and violence over drawings of Muhammad. She said in a statement after the shooting that it showed how “needed our event really was.”

In January, 12 people were killed by gunmen in an attack against the Paris office of the satirical newspaper Charlie Hebdo, which had lampooned Islam and other religions and used depictions of Muhammad. Another deadly shooting occurred the following month at a free speech event in Copenhagen featuring an artist who had caricatured the prophet.

Tens of thousands of people rallied around the world to honor the victims and defend the freedom of expression following those shootings.

Geller’s group is known for mounting a campaign against the building of an Islamic center blocks from the World Trade Center site and for buying advertising space in cities across the U.S. criticizing Islam.

When a Chicago-based nonprofit held a January fundraiser in Garland designed to help Muslims combat negative depictions of their faith, Geller spearheaded about 1,000 picketers at the event. One chanted: “Go back to your own countries! We don’t want you here!” Others held signs with messages such as, “Insult those who behead others,” an apparent reference to recent beheadings by the militant group Islamic State.

Associated Press reporter Brian Skoloff in Phoenix contributed to this report.