Two gunmen were shot dead after they opened fire outside an exhibition of Prophet Mohammed cartoons.
A bomb squad was called in to search the duo's vehicle after the pair were killed having opened fire on a security officer outside the show in Texas.
The men drove up to the Curtis Culwell Centre in the Dallas suburb of Garland as the event was scheduled to end and began shooting at a security officer, the City of Garland said.
Garland police officers returned fire and the men were killed.
Officer Joe Harn, spokesman for the Garland police department, said: "Because of the situation of what was going on today and the history of what we've been told has happened at other events like this, we are considering their car (is) possibly containing a bomb."
Police were not aware of any continuing threat and had not received any credible threats before the event.
Mr Harn said it was not clear if the shooting was connected to the event inside, hosted by the New York-based American Freedom Defence Initiative, and awarding £6,600 for the best cartoon depicting the Prophet Mohammed.
Such drawings are deemed insulting to many followers of Islam and have sparked violence around the world.
According to mainstream Islamic tradition, any physical depiction of the Prophet Mohammed - even a respectful one - is considered blasphemous.
The security officer who was shot worked for the Garland Independent School District. He was treated at a local hospital for injuries and then released.
After the shooting, about 75 attendees at the contest were escorted to another room in the conference centre. They were then taken to a separate location, and were told they could not leave until FBI agents arrived to question them.
In January, 12 people were killed by gunmen in an attack against the Paris office of the satirical newspaper Charlie Hebdo, which had lampooned Islam and other religions and used depictions of Mohammed.
Another deadly shooting occurred the following month at a free speech event in Copenhagen featuring an artist who had caricatured the prophet.
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The AFDI is known for mounting a campaign against the building of an Islamic centre close to the World Trade Centre site in New York City, and for buying advertising space in cities across the US criticising Islam.
