Federal agents for years monitored one of the two gunmen shot dead after opening fire with assault rifles at a heavily guarded Texas exhibit of caricatures of the Prophet Muhammad.

Two law enforcement officials who asked not to be named said one of the dead shooters was believed to be Elton Simpson, under surveillance since 2006 and convicted in 2010 of lying to FBI agents over his desire to join violent jihad in Somalia.

Phoenix FBI spokesman Perryn Collier confirmed agents were at the Autumn Ridge Apartments in connection with the shooting in Texas. The complex in north-central Phoenix was cordoned off and residents were evacuated for several hours in the early morning.