One of two gunmen shot dead at an event in Texas exhibiting cartoons of the Prophet Muhammad had been a terror suspect, court documents show.

Elton Simpson had been under surveillance since 2006 and was convicted in 2010 for lying about plans to go to Somalia, the files reveal.

He had shared a flat in Arizona with the person named by authorities as the other gunman, Nadir Soofi.

Both were shot after opening fire at a conference centre near Dallas.

Simpson's family has said that there was no sign he was planning to carry out such an attack.

"We are sure many people in this country are curious to know if we had any idea of Elton's plans. To that we say, without question, we did not," said a statement issued through lawyers in Phoenix.

The statement, made on behalf on unnamed family members, said his relatives were "heartbroken and in a state of deep shock".

Fine and probation

On Monday FBI agents searched Simpson and Soofi's home in Phoenix and a white van parked outside.

According to Arizona court documents published in the US media, Simpson was charged in 2010 with lying to FBI agents about planning to go to Somalia to engage in violent jihad, or holy war.

A judge found him guilty of making a false statement and he was sentenced to three years probation and a $600 (£400) fine.

The judge ruled there was insufficient evidence that the false statement involved international terrorism.

The court document also showed that Simpson had been under investigation since 2006 because of his association with an individual the FBI believed was trying to set up a terrorist cell in Arizona.

Simpson had told an informant in 2009 that it was "time to go to Somalia", adding: "We gonna make it to the battlefield."

He later said he was planning to travel to South Africa and then on to Somalia.

The incident on Sunday unfolded when a car drove into the car park of the Curtis Culwell Center in Garland, a city near Dallas, where a Muhammad Art Exhibit organised by the controversial American Freedom Defense Initiative (AFDI) was being held.

The conference included a contest that offered a $10,000 (£6,600) prize for a cartoon of the Prophet Muhammad. Depictions of the Prophet Muhammad are offensive to many Muslims.

The two men in the car opened fire with assault rifles on two security officers, Garland police said.

One of the officers, a traffic policeman, returned fire and killed both gunmen, police spokesman Joe Harn said.

The security officer who with him was shot in the lower leg. He was treated in hospital and released.

A bomb squad searched the suspects' vehicle and set off several controlled explosions. They found additional ammunition but no bombs,  Harn said.

Texas Governor Greg Abbott called the attack a "heinous crime" and said he was being briefed by state authorities.

The AFDI is run by controversial blogger and activist Pamela Geller who said she was standing up for free speech, adding: "This terrible incident reflects the need for such conferences."

One of the keynote speakers at Sunday's event was the Dutch politician Geert Wilders, an outspoken critic of Islam in Western societies.

There were widespread protests in 2006 when the Danish newspaper Jyllands-Posten published cartoons satirising the Prophet Muhammad.

In January this year, 12 people were murdered by two islamist gunmen at the French magazine Charlie Hebdo, which had published similar cartoons.

And a gathering of free speech activists in the Danish capital Copenhagen was targeted by a gunman in February, killing a film director.