Two gunmen were killed after they opened fire Sunday evening outside an event hosted by an anti-Islam group in Garland, Tex., featuring cartoons of the Prophet Muhammad, local officials said. According to the authorities, the two assailants shot a security guard and were, in turn, shot and killed by police officers.

Officials did not name the gunmen or assign a motive for the attack. A spokeswoman for the F.B.I. in Dallas said the agency was providing investigative and bomb technician assistance to the Garland police.

The City of Garland confirmed the episode in a Facebook posting.

The shooting began shortly before 7 p.m. outside the Curtis Culwell Center at an event organized by the American Freedom Defense Initiative, an anti-Islam organization based in New York.

“As today’s Muhammad Art Exhibit event at the Curtis Culwell Center was coming to an end,” the Facebook posting said, “two males drove up to the front of the building in a car. Both males were armed and began shooting at a Garland I.S.D. security officer.”