US police are searching the home of suspected gunmen who attacked a conference on cartoons of the Prophet Mohammed in Dallas.
An FBI official said agents were collecting evidence at an apartment in Phoenix, Arizona.
One suspect was identified as Elton Simpson, who had been previously investigated on suspicion of terrorism offences.
Two gunmen were shot dead after opening fire outside the contest on Sunday.
They drove to the Mohammed Art Exhibit in the Dallas suburb of Garland as the event was ending, shooting at and wounding a security officer before being killed by police.
The event was organised by the American Freedom Defence Initiative (AFDI), which is run by controversial blogger and activist Pamela Geller.
It is understood the attack was inspired by Isil. One of the keynote speakers was the Dutch politician Geert Wilders, an outspoken critic of Islam in Western societies.
Simpson (30), an American Muslim from Phoenix in Arizona, was well known to the authorities. The other unnamed suspect was thought to be his room-mate.
The exhibition's organisers had offered a $10,000 prize for the best cartoon of the Prophet Mohammed.
Mr Wilders, who is on an al-Qa'ida hit list, was among the main speakers at the event.
Texas police shot dead the two gunmen in the car park before they could begin their attack.
One of the suspects, after being initially wounded by police gunfire, was seen reaching for a backpack and was shot again and killed, said Douglas Athas, mayor of Garland.
One security guard was shot in the leg, but released from hospital hours later. Simpson died in the attack. One of the men posted on Twitter that he was a supporter of Isil just 15 minutes before the attack. It was not known whether the Twitter account was used by Simpson.
Simpson first came onto the authorities' radar in 2006, when the FBI, growing suspicious of his radical leanings, asked an informant at his local mosque to make friends with him and find out more about his intentions. Simpson was known to be connected to a man in Arizona who was attempting to set up a terrorist cell.
The informant reported that Simpson said Allah was supporting those "out there fighting non-Muslims". He praised Sharia law, and in May 2009 spoke of his wish to travel to Somalia and join al-Shabaab.
"It's time to go to Somalia, brother," he told the informant, in taped conversations played in court. "We know plenty of brothers from Somalia. We're going to make it to the battlefield. It's time to roll."
He said he was inspired by "people fighting and killing your kids, and dropping bombs on people that have nothing to do with nothing.
"You've got to fight back, and can't just be there sitting down smiling at each other."
In June 2009, he told the informant he was "tired of living under non-Muslims" and had explored the possibility of martyrdom operations. He spoke of his anger at then-president George W Bush's comments that you were "either with us or against us".
Simpson said: "You're either with us or you're with the terrorists. Bush is saying you're with us or with the Muslims."
By November 2009, he was discussing plans of how to get to Somalia, travelling first to South Africa under the pretence of studying at a madrasa, and then continuing overland.
In January 2010, three FBI agents went to his house, and questioned him about his travel plans and intention to go to Somalia - something which he denied. The FBI tried unsuccessfully to put him on the No Fly List, and then arrested him, charged with making false statements.
He was found guilty of making false statements, but the judge ruled that the government did not provide enough evidence to support charges of international and domestic terrorism. He was sentenced, in March 2011, to three years probation. (© Daily Telegraph, London)