Anti-terror experts have raised doubts about a claim by the Islamic State (IS) group that it was responsible for the attack on a Texas cartoon contest that featured images of the Prophet Mohammed.
The experts say IS has a history of asserting involvement in attacks in which it had no operational role - suggesting the two gunmen could have carried out their own lone wolf-style strike before being shot dead at the scene of Sunday's shooting in the Dallas suburb of Garland.
The gunmen were identified as Elton Simpson and Nadir Soofi, both Americans who lived in Phoenix, Arizona. US government authorities had been scrutinising Simpson's social media presence recently, but had no indication he was plotting an attack, said one federal official familiar with the investigation.
Michael McCaul, chairman of the House of Representatives Homeland Security Committee, said a Twitter account linked to Simpson included images of US-born Anwar al-Awlaki, a radical cleric killed in a CIA drone strike in Yemen.
Among the hashtags used by the account was "texasattack" and one of the final tweets was: "May Allah accept us as mujahideen (holy warriors)."
"Was he on the radar? Sure he was," Mr McCaul said from Turkey, where he was leading a congressional delegation.
The evidence did not indicate the attack was directed by IS, "but rather inspired by them", said McCaul, who was briefed on the investigation by federal law enforcement officials. "This is the textbook case of what we're most concerned about."
A federal source said authorities had an open investigation into Simpson at the time of the shooting and authorities would be studying the contacts the men had before the shooting, both with associates in the US and abroad, to determine any additional terror-related ties.
White House press secretary Josh Earnest said US officials were working to counter terrorist efforts to use social media to radicalise individuals in the United States.
IS recently urged those in the United States, Europe and Australia who cannot safely travel to fight in Syria and Iraq to carry out jihad in the countries where they live. An audio statement on the extremist group's Al Bayan radio station called the men "two soldiers of the caliphate".
The shooting appeared to be another example of a "do-it-yourself" jihadist whose plots are often hard for law enforcement and intelligence agencies to stop, said Mitchell Silber, executive managing director for K2 Intelligence and former director of intelligence analysis for the New York City police department.
"It's very tough to detect in advance, which means we are and will continue to be susceptible to lone actors who don't give us much warning to thwart them," he said.
The cartoon contest had been expected to draw outrage from the Muslim community. According to mainstream Islamic tradition, any physical depiction of Mohammed - even a respectful one - is considered blasphemous, and drawings similar to those featured at the Texas event have sparked violence around the world.
Simpson, 31, and 34-year-old Soofi were wearing body armour and one of the men shot a security officer in the leg before a single Garland police officer fired on the gunmen. After his initial shots, nearby SWAT officers also fired, authorities said.
Simpson was arrested in 2010 after being the focus of a four-year terror investigation, but despite amassing more than 1,500 hours of recorded conversations, including Simpson's discussions about fighting non-believers for Allah and plans to link up with "brothers" in Somalia, the government prosecuted him on only one minor charge - lying to a federal agent.
He was sentenced to three years of probation and ordered to pay 600 dollars (£400) in fines and court fees.
The men were described as amicable and quiet and were sometimes seen feeding stray cats outside their apartment complex.
Simpson had worshipped at the Islamic Community Centre of Phoenix for about a decade, but he stopped attending over the past two or three months.
The centre's president, Usama Shami, said Simpson would play basketball with mosque members and was involved with the community. Soofi owned a nearby pizza business and would come in to pray occasionally.
"They didn't show any signs of radicalisation," Mr Shami said.
Soofi's mother Sharon told The Dallas Morning News that her son may have somehow snapped.
"The hard thing is to comprehend is why he would do this and leave an eight-year-old son behind," said Ms Soofi, who now lives in a small town south west of Houston.
A University of Utah spokeswoman said Soofi was an undergraduate pre-medicine student enrolled from autumn 1998 to the summer of 2003. He did not earn a degree.
In a statement released by Phoenix law firm Osborn Maledon, Simpson's family said it was "struggling to understand" what happened.
"We are sure many people in this country are curious to know if we had any idea of Elton's plans," the statement said. "To that we say, without question, we did not."
Ben Venzke, who has been tracking terrorist groups for two decades, said jihadists have shifted their tactics to include not only major targets like al Qaida's attack on the World Trade Centre, but small ones that are more easily accessible.