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

The heavily armed gunmen who attacked a cartoon contest near Dallas over the weekend were probably inspired by the Islamic State, according to U.S. officials, who cautioned that they have so far seen no indication that the assailants were directed by the group.

Law enforcement officials said Tuesday that they were still analyzing the shooters’ electronic devices, including phones and computers, to determine whether others may have been involved in the plot or encouraged it.

The men — Elton Simpson, 30, a Muslim convert, and his roommate, Nadir Soofi, 34 — are not known to have had formal ties to the Islamic State, officials said, but the group’s propaganda could have fueled their decision to attack the gathering in Garland, Tex.

In online postings, the group’s followers had drawn attention to the cartoon contest, intended to ridicule the Islamic prophet Muhammad. On Tuesday, a Syria-based radio station operated by the Islamic State asserted responsibility for the attack.

“We say to the America, the defender of the cross, what’s coming will be even worse,” the station said, according to a translation by the SITE Intelligence Group, which tracks online postings from Islamist militant organizations.

While the FBI closely monitors U.S. citizens who have tried to travel overseas to join the Islamic State, officials have also seen a growing number of cases in which supporters of the group or other radical Islamist causes have planned domestic attacks.

The FBI has recently charged several suspects with attempting to stage attacks in the United States in the name of the Islamic State. In March, officials arrested two men in Illinois, one of whom has been accused of plotting to attack a military installation in hopes of killing scores of people. The FBI also charged two women in New York with conspiring to build explosive devices to detonate in the United States.

Seeking a ‘lone wolf’

Simpson and Soofi had traveled roughly 1,000 miles from Phoenix to Garland in time for the Sunday event, called the Muhammad Art Exhibit, which promised a $10,000 prize for the best cartoon depicting the founder of Islam. Both were killed after opening fire on a security guard, wounding him, outside the arena where the event was being held.

Law enforcement officials said the attackers were armed with six weapons, including four handguns and two semi-automatic assault rifles.

The FBI and local authorities had expanded security measures before the event. The president of the New York-based group behind the contest, Pamela Geller, recently sponsored an advertisement campaign that placed anti-Muslim posters in major U.S. cities.

U.S. officials are trying to determine whether Simpson and Soofi had ties to the Islamic State or could have been responding to calls by the group’s followers. In late April, Simpson was apparently in contact with a Twitter user who urged others to carry out a “lone wolf” attack directed at the contest, according to SITE.

Simpson had been arrested in 2010 and charged with lying to the FBI about plans to travel overseas to wage violent jihad in Somalia. He was convicted the following year on a lesser charge and sentenced to three years’ probation.

The mother of one of the gunmen in Sunday’s 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)

The FBI had an active investigation focused on him again in recent months, tracking his Twitter feed and conducting intermittent surveillance. But officials said his willingness and ability to carry out a violent attack caught them off guard.

“We considered him a keyboard jihadi — a kid who would rant, not someone who would do something dumb and dangerous,” said one law enforcement official, who spoke on the condition of anonymity because the investigation is ongoing. The official said Simpson did not seem to pose a clear and immediate danger.

Soofi attended the University of Utah as a pre-med student from 1998 to 2003 but is believed to have lived in Pakistan for several years in the 1990s.

Friends and relatives of the two men were trying to come to grips with what happened in Texas.

Usama Shami, president of the Islamic Community Center of Phoenix, said Simpson began worshiping at the mosque about 10 years ago.

“He was a young guy, full of energy,” Shami said. “He was very nice, very cordial, very respectful. He wanted to know more and get involved in the community.”

No signs of extremism

Shami said there were no outward signs that Simpson or Soofi had been radicalized.

“They did not engage in anything controversial. They did not outwardly come and say they support ISIS or they were thinking about going and attacking somebody,” he said, using an acronym for the Islamic State. “This stuff, they never discussed.”

After Simpson was arrested, he began attending the mosque less frequently, Shami said, adding that the FBI’s reliance on an informant who attended the mosque strained the community’s relationship with law enforcement.

“I always thought the relationship with law enforcement should be built on trust,” he said.

It’s not clear what Simpson was doing before he and Soofi made plans to attack the cartoon contest.

According to the Dallas Morning News, Soofi’s mother, who lives in the Houston area, said her son was religious and politically active but didn’t espouse violence.

“He was raised in a normal American fashion,” she told the newspaper. “Yes, he was very politically involved with the Middle East. Just aware of what’s going on. I don’t know if something snapped or if Elton Simpson was just working on him.”

In Phoenix, Nadir Soofi ran a restaurant and later a carpet-cleaning business.

On Tuesday, the brother of the dead gunman posted a picture on his Facebook page of Soofi: a bearded young man wearing sunglasses and a skullcap.

Ali Soofi, who has been interviewed by investigators and is not considered a person of interest, didn’t respond to messages sent to his Facebook account.

An earlier version of this story incorrectly stated that the gunmen in the attack were armed with AR-15 semiautomatic rifles.

Goldman reported from Washington. Scott Higham and Julie Tate in Washington and Tim Craig in Islamabad, Pakistan, contributed to this report.