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By David Schwartz PHOENIX (Reuters) - The Islamic State militant group claimed responsibility on Tuesday for an attack on a Texas exhibit of caricatures of the Prophet Mohammad in which the two gunmen were killed. The Syria- and Iraq-based Islamic State claimed responsibility on its official online radio station, saying "two soldiers of the caliphate" carried out the attack on Sunday in Garland, Texas. Experts warn that militant groups have been known to claim credit for attacks in which they were not involved. U.S. government sources close to the case have said investigators were scouring electronic communications sent and received by the dead gunmen, roommates Elton Simpson and Nadir Soofi, of Phoenix, for evidence of contacts between them and militant groups overseas, most notably Islamic State.