The Islamic State has claimed responsibility for the shooting in Garland, Texas. The Washington Post's Adam Goldman describes the extent of the terror group's reach in the United States. (Jhaan Elker/The Washington Post)

Investigators worked Monday to retrace the steps of two Phoenix men who were gunned down by police after traveling 1,000 miles across the Southwest to open fire on a provocative exhibit that lampooned the prophet Muhammad.

Authorities identified the assailants in the Sunday night attack as Elton Simpson, 30, a Muslim convert whom the FBI had previously targeted in a terrorism investigation, and his roommate, Nadir Soofi, 34.

Their bodies were left lying in the street for more than 12 hours near the west entrance of the Curtis Culwell Center, an arena and exhibition hall in this suburb northeast of Dallas, as investigators cautiously searched their vehicle for bombs.

No explosives were found, although the gunmen wore body armor and had been carrying extra ammunition and suitcases in their car, said Joe Harn, a spokesman for Garland police. An unarmed security guard was shot in the leg. He was treated at a hospital and released.

Authorities were cautious about ascribing a motive, but there seemed little doubt that the shooting marked a collision of religious fundamentalists and free-speech provocateurs in a replay of jihadist attacks in Paris and Copenhagen inspired by cartoons of the Islamic prophet. Drawings of Muhammad are widely considered blasphemous by Muslims.

From staging a Texas competition to draw the prophet Muhammed to the protests against the so-called 9/11 mosque, Pamela Geller has led anti-Muslim campaigns for years. So who is she? (Jorge Ribas/The Washington Post)

The inflammatory Texas event, dubbed the Muhammad Art Exhibit, promised a $10,000 prize for the best cartoon depicting the founder of Islam. The contest was sponsored by the American Freedom Defense Initiative, a New York-based organization that has been labeled an anti-Muslim “hate group” by the Southern Poverty Law Center, a civil rights network that tracks domestic extremists.

Pamela Geller, the group’s president, is known for blogging conspiracy theories and incendiary rhetoric against what she calls the “Islamization” of the United States. The keynote speaker at Sunday’s event was Geert Wilders, a firebrand Dutch parliamentarian who has sought to ban the Koran in the Netherlands and has been marked for assassination by al-Qaeda and its allies.

The event’s organizers said they had wanted to make a stand for free speech following the January attack by armed jihadists against the Paris newsroom of Charlie Hebdo, a satirical newspaper that routinely made fun of Islamic extremists.

“Our Judeo-Christian culture is far superior to the Islamic one,” Wilders said in his speech Sunday, shortly before the gunfire erupted, according to a text of the address posted on his personal Web site. “I can give you a million reasons. But here is an important one: We have got humor and they don’t.”

The exhibit’s sponsors were ready for conflict. They had hired dozens of security guards and off-duty police officers — including a heavily armed SWAT team — to protect the approximately 200 attendees at the invitation-only event. Some participants appeared to revel in the fact that the show was drawing worldwide attention from jihadists, who posted social media messages urging followers in the United States to launch an attack in Garland.

Enter the two gunmen, whom Garland police said drove up to a police car that was blocking an entrance to the exhibition hall and opened fire with assault rifles. Both men were quickly killed by one of the hired guards — a local traffic patrol officer whose name has not been released — who returned fire with a semiautomatic pistol.

“He did a very good job and he probably saved lives,” said Harn, the police spokesman.

The mother of one of the gunmen in Sunday's Mohammad cartoon attack says her son was not a violent person. Sharon Soofi said she doesn’t blame police for shooting Nadir but is left feeling “empty.” (Reuters)

Police established a large security cordon around the Curtis Culwell Center, located on the north side of Garland on a broad boulevard near a high school, a Sam’s Club and a Walmart Supercenter. Beyond the yellow police tape blocking the boulevard, FBI agents could be seen wandering Monday afternoon near the scene of the shooting. The suspects’ black sedan remained in a parking lot, its trunk blown open by a police bomb squad.

In Phoenix on Monday, police and federal agents likewise combed a low-rise apartment complex where Simpson and Soofi lived. Parts of the complex were evacuated a few hours before dawn as specialists searched the pair’s home for booby traps, but they found none.

Carolyn Hutson, a 48-year-old neighbor, described Simpson as aloof and somewhat evasive. He was usually dressed in a long robe and cap, she said.

“He seemed very guarded, kept to himself,” Hutson said. “He was very quiet. He would stand out on his patio and talk on the phone.”

Other residents said that another young man appeared to live in the apartment with Simpson and Soofi. Neighbor Tim Rains, 57, said he would often see them working on their cars in the parking lot. Rains said he had little direct contact with them except for one occasion last year when Simpson saw Rains faltering from a heart condition and came over to help him back inside his apartment.

Court documents show that Simpson was born in Illinois and converted to Islam at a young age. The government began investigating him in 2006, recording conversations between him and a paid informant.

In May 2009, according to a federal court document, Simpson told an FBI informant: “It’s time to go to Somalia, brother.” He added: “It’s time. I’m tellin’ you, man. We gonna make it to the battlefield. . . . It’s time to roll.”

Simpson was arrested by the FBI in January 2010 after a lengthy investigation and charged with lying to agents in connection with terrorism. Authorities suspected he was trying to fly to Somalia, but Simpson claimed at the time he intended to travel to South Africa to go to school to study Islam.

Following a bench trial, a judge dropped the terrorism allegations, citing insufficient evidence. The judge, Mary H. Murguia, said in March 2011 that the government had failed to prove that Simpson intended to wage violent jihad in Somalia.

Murguia reduced the charge to making a false statement to federal officials and sentenced Simpson to three years of probation. Authorities also returned his passport, which they had confiscated after his arrest.

Simpson’s lawyer, Kristina Sitton, described him as a particularly devout Muslim.

“He didn’t seem to me to be any threat to anybody,” Sitton said in a telephone interview. “He seemed to be very kind but entrenched in Islam. He wouldn’t shake my hand.”

She said that after he was sentenced to probation, Simpson called her saying that he had tried to board a domestic flight and was told he could not fly. Sitton said she believed he was on a government no-fly list.

Soofi, Simpson’s roommate, ran a carpet-cleaning business he started last year. His Facebook account focused on Israel’s treatment of Palestinians in the occupied territories as well as on police brutality in the United States.

Soofi attended the University of Utah as a pre-med student from 1998 to 2003, according to a spokeswoman for the school. He left the school in the summer of 2003 without having earned a degree, she said.

In recent months, the FBI had begun monitoring Simpson again because of postings on social media in which he praised the Islamic State and other radical groups, officials said. It was unclear to what degree he and his movements may have been under surveillance.

Minutes before the shooting, someone posted a message from a Twitter account linked to Simpson: “May Allah accept us as mujahideen.” Appended was a hashtag with the label ­“#texasattack.”

The Twitter account features an image of Anwar al-Awlaki, a U.S.-born imam who became an influential propagandist with al-Qaeda’s affiliate in Yemen. Awlaki was killed in a U.S. drone strike in Yemen in 2011.

The Council on American-Islamic Relations, a leading civil rights and advocacy group for Muslims, condemned Sunday’s shooting. It called the attack “more insulting to our faith than any cartoon, however defamatory. Bigoted speech can never be an excuse for violence.”

The winner of the Texas cartoon contest was Bosch Fawstin, a New York native and the son of Albanian Muslim immigrants. His entry featured a scowling, turbaned Muhammad saying, “You can’t draw me!”

At the bottom of the cartoon was the illustrator’s written response. “That’s why I draw you,” it said.

Richter, a freelance writer, reported from Garland. Evan Wyloge, of the Arizona Center for Investigative Reporting in Phoenix, and Mark Berman and Julie Tate in Washington contributed to this report.