Iraq ? A bomb exploded inside Sadr City’s district council building Tuesday, killing 10 people, including four Americans working to restore local government and services in the former Shiite militia stronghold.

Iraqi officials said it appeared to be an inside job, and suspicion fell on the headquarters’ Shiite guard force. The blast was the second deadly attack to strike Americans promoting municipal governments in as many days.

The attack comes as U.S. military and civilian officials step up efforts to take advantage of a sharp drop in violence to promote the local administration and restore services in Sadr City and other areas. Failure to do so could allow Sunni and Shiite extremists to regain a foothold, U.S. commanders believe.

The increased presence of the Americans in local communities has made them more vulnerable to attacks. But U.S. commanders believe that’s key to a strategy that has helped drive down the levels of violence to the lowest point in more than four years.