Last Updated May 4, 2015 2:03 PM EDT
GARLAND, Texas -- Two gunmen were killed Sunday after opening fire on a security officer outside a provocative contest for cartoon depictions of Prophet Muhammad in Texas, and a bomb squad was called in to destroy their vehicle as a precaution, authorities said.
The men drove up to the Curtis Culwell Center in the Dallas suburb of Garland as the contest was scheduled to end and began shooting at a security officer, the City of Garland said in a statement. At least one Garland police officer returned fire, killing the men seconds after the attack began. Officials later credited the officer for saving lives by preventing the attack from entering the building.
The attackers have been identified as Elton Simpson and Nadir Soofi.
CBS Dallas reports Simpson was the subject of a previous terror investigation. He was charged in 2010 with making false statements to federal agents in regards to a Somalia trip. He was traveling there "for the purpose of engaging in violent jihad," according to the charges, and was found guilty in March 2011 of making a false statement. But the court ruled that there was "insufficient evidence to support that the false statement 'involved' international terrorism."
Simpson was given three years probation and a $600 fine, and the case was closed in September 2011.
FBI officials told CBS affiliate KPHOI in Phoenix the gunmen are from the Phoenix area. A search warrant has been executed for their apartment.
Soofi has been identified as Simpson's roommate. A senior law enforcement officials told CBS News senior investigative correspondent Pat Milton that they do not believe Soofi had been on law enforcement radar prior to this shooting incident.
Garland Mayor Douglas Athas said one of the gunmen was shot immediately by police, and the other was shot and killed when he reached for a backpack, leading officers to fear the men may have brought explosives, reports The Los Angeles Times.
It appears the suspects' vehicle was detonated out of precaution following the shooting.
"Because of the situation of what was going on today and the history of what we've been told has happened at other events like this, we are considering their car (is) possibly containing a bomb," Officer Joe Harn, a spokesman for the Garland Police Department, said at a news conference.
Police are not aware of any ongoing threat and had not received any credible threats before the event, Harn said.
Law enforcement officials said that, prior to the attack, messages concerning radical Islamic viewpoints were posted on social media pages from two accounts. One comment on Twitter even used the hashtag #TexasAttack.
The Los Angeles Times reports one tweet, sent at 6:35 p.m., stated: "May Allah accept us as mujahideen."
Attendees at the contest didn't get word about the shooting until about 6:50 p.m. It is not clear whether the social media messages are linked directly to the attack.
Officials have not yet officially connected the shooting the event inside, a contest hosted by the New York-based American Freedom Defense Initiative that would award $10,000 for the best cartoon depicting the Prophet Muhammad.
Such drawings are deemed insulting to many followers of Islam and have sparked violence around the world. According to mainstream Islamic tradition, any physical depiction of the Prophet Muhammad - even a respectful one - is considered blasphemous.
Simpson, according to law enforcement officials, moved to Phoenix and converted to Islam at a young age. The FBI recorded conversations between Simpson and an informant and the two frequently discussed jihad and the importance of going overseas to conduct jihad.
In early 2010, FBI agents stopped Simpson from traveling to South Africa, where he was allegedly going to study Islam, but court documents state that the government believed this was a cover for attempting to reach Somalia and wage jihad. When he was interviewed, Simpson denied the accusations.
Ultimately, he was only convicted of making false statements to the FBI, which is a felony.
The Curtis Culwell Center, a school-district owned public events space, was evacuated earlier Sunday, as were some surrounding businesses. Police blocked off a large area around the center Sunday night. There was a heavy police presence, and police helicopters circled overhead as bomb squads worked on the car.
A series of controlled detonations could be heard late Sunday night and early Monday morning, reports CBS Dallas.
Authorities did not immediately remove the bodies of the gunmen, because they were too close to the suspects' vehicle before it was blown up. Officials said later a bag of ammunition was found inside the vehicle.
The security officer who was shot worked for the Garland Independent School District, Harn said. He was treated at a local hospital for injuries and had been released Sunday night.
"Texas officials are actively investigating to determine the cause and scope of the senseless attack," Texas Gov. Greg Abbott said in a statement issued late Sunday.
Harris Zafar, Vice President of the Ahmadiyya Muslim Youth Association, told CBS Dallas via email, "People need to hear a strong Muslim voice condemning this insanity."
Ahead of Sunday's conference, the Council on American-Islamic Relations said the meeting was a "clear attempt to bait the Muslim community," reports CBS News correspondent Omar Villafranca.
The event featured speeches by Pamela Geller, president of the AFDI, and Geert Wilders, a Dutch lawmaker known for his outspoken criticism of Islam. Wilders received several standing ovations as he quoted former President Ronald Reagan and Texas founding father Sam Houston.
"Muhammad fought and terrorized people with the swords. Today, here in Garland, we fight Muhammad and his followers with the pen. And the pen, the drawings, will prove mightier than the sword," Wilders said during 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.
Geller said during the event that her group had spent thousands of dollars on private security guards, several of whom could be seen standing on stage. Wilders left immediately after his speech, and Geller was not seen by an Associated Press reporter inside the hall after the shooting.
After the shooting, about 75 attendees were escorted by authorities to another room in the conference center. A woman held up an American flag, and the crowd sang "God Bless America."
They were then taken to a separate location, where they were held for about two hours until they were questioned briefly by FBI agents who took brief written statements from most people. Police officers drove some attendees to their homes and hotels, and arranged hotel rooms for others whose cars remained under lockdown.
A CBS Dallas reporter and camera team were inside when the gunfire broke out, and were among those put on lockdown.
The FBI is providing investigative assistance and bomb technician assistance, Dallas FBI spokeswoman Katherine Chaumont said.
Johnny Roby, of Oklahoma City, Oklahoma, was attending the conference. He told the AP he was outside the building when he heard around about 20 shots that appeared to be coming from the direction of 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. Though it remained unclear several hours after the shooting whether it was related to event, she said Sunday night that the shooting showed how "needed our event really was."
Local Muslims had condemned the event, but religious leaders had urged them to not protest it. One prominent local Imam tweeted afterwards he was glad his community avoided it.
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.
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 the Islamic State of Iraq and Syria.