Police have said an officer was also fired at outside the Curtis Culwell Centre in Garland, Texas, during a "family event" where caricatures of the Prophet Muhammad were being displayed.
The city of Garland said in a statement posted on its Facebook page Sunday night (local time) that two men drove up to the Curtis Culwell Center and began shooting at a security officer.
As today’s Muhammad Art Exhibit event at the Curtis Culwell Center was coming to an end, two males drove up to the front...
Posted by City of Garland, Texas Government on Sunday, May 3, 2015
The statement doesn't say whether the shooting was related to the event.
The building was placed on lockdown by a SWAT team with hundreds of attendees still inside after multiple gunshots were heard.
An officer dressed in SWAT gear took the stage toward the end of the event, to say a shooting had occurred.
A reporter with WFAA in Dallas tweeted that people were evacuated while singing "The Star-Spangled Banner":
Ppl evacuated singing patriotic songs. @wfaachannel8 pic.twitter.com/8xOWS9isv1
— Jobin Panicker (@jobinpnews) May 4, 2015
Two suspects were said to have approached the area, possibly with explosives, before firing.
A local police-themed blog that posts information coming from public scanner radio said a suspect had been spotted at a nearby store with a hand grenade, and that the expo centre and surrounding businesses had been evacuated.
"There are multiple reports that two explosives have been placed in the area of Curtis Caldwell Centre," the Rowlett/Sachse Scanner wrote on its Facebook page.
UPDATE ON THE POLICE INCIDENT:Two subjects opened fire on a Garland ISD officer. The two middle eastern subjects who...
Posted by Rowlett/Sachse Scanner on Sunday, May 3, 2015
Ms Geller had tweeted from her account: "#garlandshooting cop shot - two suspects dead, awaiting bomb squad for possible explosives at our free speech event #sharia."
#garlandshooting cop shot - two suspects dead, awaiting bomb squad for possible explosives at our free speech event #sharia
— Pamela Geller (@PamelaGeller) May 4, 2015
Johnny Roby was attending the conference and told AP he was outside the building when he heard about 20 shots that appeared to be coming from the direction of a passing car.
The event was set up by the American Freedom Defense Initiative and had been described by opponents as an attack on Islam.
The organizers believed they were exercising their freedom of expression. Geller called the shootings "war on free speech".
"What are we going to do? Are we going to surrender to these monsters?" she wrote on her website.
"The war is here."
Geller's group is known for mounting a campaign against the building of an Islamic center blocks from the World Trade Center site and for buying advertising space in cities across the U.S. criticizing Islam.
According to the Dallas Morning News, the contest, which was offering a top prize of $10,000, had received about 350 entries depicting Muhammad.
Visual representation of the prophet is deemed extremely offensive by Muslims.
The officer, who was shot in the leg, is expected to survive.
Dutch member of parliament and leader of the far-right Party for Freedom, Geert Wilders, had just delivered the keynote address inside the exhibition when the shooting occurred.
Wilders also commented on the incident, saying: "Shots fired at Garland Mohammed cartoon free speech event. I just left the building after speeching. #garlandshooting."
Shots fired at Garland Mohammed cartoon free speech event. I just left the building after speeching. #garlandshooting
— Geert Wilders (@geertwilderspvv) May 4, 2015