A roadside bomb went off outside a major security headquarters in Gaza City on Friday, in an escalation of attacks on symbols of Hamas's rule. No one was wounded in the blast, and there was no immediate claim of responsibility. Hamas blamed Palestinian Authority President Mahmoud Abbas's Fatah group, whose forces Hamas routed in June to wrest control of Gaza. But Fatah denied responsibility. Security forces in Gaza described the explosion as the largest in a wave of recent Palestinian bombings against Hamas targets in Gaza.