Last Updated May 4, 2015 11:06 PM EDT
Even before the threatening tweets, police in Garland, Texas, were preparing for the possibility of an attack on an event that had offered to reward people for their cartoon depictions of the Prophet Muhammad.
Their cautiousness ended up being well-founded, as the only people killed in the apparent attack were the gunmen themselves. A security guard was shot in the leg, but he was released from the hospital the same day.
There have been two deadly attacks in Europe this year alone perpetrated by terrorists who claimed they were avenging the drawings of Islam's main prophet. According to mainstream Islamic tradition, any physical depiction of the Prophet Muhammad -- even a respectful one -- is considered blasphemous.
"What's surrounded these type of events in other parts of the world, additional security was added to this event," Joe Harn with the Garland Police Department told CBS Dallas. "Luckily, it was."
During a Monday news conference, Harn added that $10,000 was spent on extra security for the art event, and officials started planning for potential issues months ago.
Authorities credited an off-duty officer working security at the event with saving lives by killing the gunmen. The two dead gunmen have been identified as Elton Simpson and Nadir Soofi, roommates from Phoenix.
Harn said Monday the two men stopped at a parking lot entrance blocked by a police vehicle. They came out of their car armed with assault weapons and began firing on the off-duty Garland officer and an unarmed security guard who also were getting out of their vehicle to question the men, Harn said.
The gunmen were wearing body armor, he said, and one shot the guard in the leg. The officer returned fire and struck both men, killing them. The guard was treated for his injury at a hospital and released.
Investigators later found ammunition in the car the gunmen drove to the scene.
Law enforcement officials have so far not directly linked the Muhammad cartoon event to Simpson and Soofi's motivation for the attack, but Harn said, "obviously they were there to shoot people."
CBS Dallas reports Simpson was the subject of a previous terror investigation.
According to trial testimony, Simpson is an American Muslim who became the subject of a criminal investigation in 2006 because of his association "with an individual whom the FBI believed was attempting to set up a terrorist cell in Arizona," U.S. District Judge Mary H. Murgia said in her order convicting Simpson.
"I'm telling you, man, we can make it to the battlefield," Simpson said in May 2009, according to a recording of him and an FBI informant disclosed during Simpson's trial. "It's time to roll."
The FBI said Simpson had reserved a flight to South Africa for Jan. 15, 2010. He was arrested the day before the flight. Prosecutors alleged that the false statement involved terrorism, but Murgia's order said prosecutors hadn't proved that part of the allegation.
Another federal judge later sentenced Simpson to three years of probation. Simpson was also given a $600 fine, and the case was closed in September 2011.
The apartment the roommates shared was believed to be Simpson's. 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.
A resident of the Phoenix apartment complex said the two men who lived in the unit being searched in the investigation into the Texas shooting largely kept to themselves, but that one was friendly on occasion.
Bob Kieckhaver was among the residents of the Autumn Court Apartments who were evacuated for about nine hours from units near the one being searched.
Kieckhaver said one of the men, who had a beard and wore a Muslim prayer cap, spent time working on a black Chevy that was up for sale about two months ago. Kieckhaver said that man was quiet, while the second man who lived in the apartment would greet other residents at the mailbox. He said both men would feed stray cats on a patio.
In a statement released late Monday by Phoenix law firm Osborn Maledon, Simpson's family said it is "struggling to understand" how the incident happened.
"We are sure many people in this country are curious to know if we had any idea of Elton's plans," the statement says. "To that we say, without question, we did not."
The statement, which does not identify the relatives, also says the family is "heartbroken and in a state of deep shock" and sends prayers to everyone affected by this "act of senseless violence," especially the security guard who was injured.
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., right around the time of the shooting, 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.
The event the two attacked was called the First Annual Muhammad Art Exhibit and Contest. Approximately 200 people were in attendance when the shooting incident began.
It was organized by Pamela Geller, a New York activist who rails against Islam with such ferocity that one of the nation's top civil rights groups lists her in its "extremist files."
The event featured speeches by 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.
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.
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 groupthe Islamic State of Iraq and Syria.