Image copyright Reuters Image caption Police have spent several hours inspecting the gunmen's car for explosives

Two gunmen have been shot dead after opening fire outside a conference on cartoons of the Prophet Muhammad in a suburb of Dallas, US police say.

They drove to the Curtis Culwell Center in Garland as the event was ending, and began shooting at a security officer before being killed by police.

The bomb squad has been called in to search their vehicle for explosives.

The event, organised by a group critical of Islam, included a contest for drawings of the Prophet.

One of the keynote speakers was the Dutch politician Geert Wilders, an outspoken critic of Islam in Western societies.

Security had been tight, but Garland police said there had been no credible threats in advance, and it was not immediately clear if the shootings were related to the event.

ABC news reported that one of the suspects had been identified as Elton Simpson, an Arizona man who had been previously investigated on suspicion of terrorism offences.

Officials believe that he sent several Twitter messages before the Sunday's shooting, including one with the hashtag #texasattack. The Twitter account was later suspended.

Media playback is unsupported on your device Media caption Eyewitness Stephen Perkins: ''Security was very tight''

About 200 people had been attending the Muhammad Art Exhibit when, shortly before it was due to finish at 19:00 (00:00 GMT), they were told of a shooting outside.

One woman at the conference was leaving the building when she said she heard "quick shots" from what she thought was a machine gun.

"Police started running everywhere, they quickly told us to run as fast as we could to get back in the building," she said.

They and people from nearby buildings were later evacuated.

The "first suspect was shot immediately," Garland Mayor Douglas Athas told CNN. "The second suspect was wounded and reached for his backpack. He was shot again."

Security officer Bruce Joiner was taken to hospital after being shot in the ankle, but was later released.

Local police said earlier that they had not been yet able to identify the gunmen, whose bodies remained by the car while bomb experts inspected it for explosives.

Joe Harn, a spokesman for the Garland Police Department, warned that the inspection is "a very slow, tedious operation that goes on".

Image copyright AP Image caption The conference centre was locked down after the shooting

Image copyright AP Image caption Dutch politician Geert Wilders released a picture of himself with police in SWAT gear which he said was taken just before the attack

Sunday's event was organised by the American Freedom Defense Initiative (AFDI), which is run by controversial blogger and activist Pamela Geller and is listed as an anti-Muslim group by the Southern Poverty Law Center, a civil rights group.

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

The Curtis Culwell Center - a school-district owned building for public events - was at the centre of controversy in January, after it was picketed by hundreds of protesters angry that it was hosting an event to raise funds for the building of a local Islamic centre.

Mayor Athas acknowledged there had been a lot of local concern about Sunday's event "which is why we had heightened security in the area".

Ms Geller said she was standing up for free speech, adding: "This terrible incident reflects the need for such conferences."

"We are continually abridging our freedoms so as not to offend savages," she said. Geller denied she was anti-Muslim, only "anti-jihad".

In a text message to Dutch media, Geert Wilders said Sunday's incident had been an "unacceptable" attack on freedom of expression.

"The only fitting response is: to carry on. But it's not easy," he told Dutch public broadcaster NOS.

Depictions of the Prophet Muhammad are offensive to many Muslims.

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.

Image copyright Reuters Image caption Ms Geller is an outspoken critic of Islam

Pamela Geller: America's controversial blogger

A staunch critic of Islam since 2005, she rose to prominence in 2010 through her online opposition to Park 51, a planned Muslim community centre in Lower Manhattan close to the World Trade Center site

Heads the American Freedom Defense Initiative (AFDI), which has also caused controversy by buying advertising space on buses in US cities, criticising Islam

The 56-year-old describes herself online as a free-speech activist, but her critics denounce her as a "bigot"

She insists the focus of her criticism is chiefly against radical Islam, but has been quoted as saying that "Islam is the most anti-semitic, genocidal ideology in the world"

Speaking of her role in organising the Muhammad Art Exhibit in Garland, she said: "We draw Muhammad because we are free... We draw Muhammad because our unalienable rights are enshrined in the First Amendment."

Pamela Geller: America's 'bigoted blogger'