One of a pair of gunmen shot dead at a Texas exhibition of cartoons of the Prophet Mohammed was a terror suspect, according to court documents.

Elton Simpson had been under surveillance since 2006, and in 2010 was convicted after lying to FBI agents about planning to travel to Somalia, where Islamic fundamentalist terror group Al Shabaab is fighting, to wage jihad, court documents reveal.

The second gunman was named as Nadir Soofi, who Simpson shared a flat with in Arizona.

Soofi had no known criminal record.

Both men were shot dead after opening fire at the exhibition at a conference centre outside Dallas, at an event organised by the American Freedom Defence Initiative, a right wing anti-Islam organisation.

Authorities said that they monitored his social media, presence, and were investigating whether the attack may have been instigated by Isis, the Islamic extremist organisation that controls territory in Syria and Iraq.

In a series of Tweets, Simpson alluded to his support for Isis.

Simpson's family said that there was no indication he was planning 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 the collective statement, relatives said they were "heartbroken and in a state of deep shock".

Soofi's mother Shirley Dromgoole of Garwood, Texas, told NBC station KPRC that Soofi, 34, a divorced father, "was a good boy every time I saw him" and that he must have been under the spell of Simpson.

After his 2010 conviction, Simpson was sentenced to three years probation and charged $600 (£400).

The document also showed that Simpson had been under surveillance since 2006 after associating with an individual who was allegedly attempting to set up a terror cell in Arizona.

He allegedly told an FBI informant in 2009 that it was "time to go to Somalia" and "we're going to make it to the battlefield".

On Monday, FBI agents searched the pair's apartment in Phoenix, evacuating residents from the complex and sealing it off for several hours.

On Sunday, May 4, both men drove up behind a conference centre in Garland, where 200 people were attending the Draw Mohammed event, which organisers claim was held in defence of freedom of speech.

The two men got out of the car and opened fire on a police officer guarding the event, who returned fire, killing them both.

A security guard was wounded in the exchange.