Hillah, Iraq ? In the deadliest single strike since the fall of Saddam Hussein, a suicide car bomber attacked mostly Shiite police and National Guard recruits lined up for physical exams at a medical clinic Monday, killing 115 and wounding 132 there and at a nearby market.

The bombing presented the boldest challenge yet to Iraq’s efforts to build a security force that can take over from the Americans.

The explosion in Hillah, a largely Shiite Muslim town about 60 miles south of Baghdad, was so powerful that the only thing remaining of the bomber’s car was the twisted wreckage of the engine block.

Dozens of people stepped through small lakes of blood that pooled on the street to retrieve shattered limbs, severed feet and hands.

The bombing comes at a time when the Sunni Arab insurgency is trying to disrupt the formation of a new government set to be led by majority Shiites for the first time in modern history. Iraqi forces are eventually supposed to take over responsibility for security — the key to Washington’s exit strategy — but they remain under-equipped, ill-prepared to fight insurgents and often make easy targets.

The Shiites have refrained from striking back — mostly at the behest of their most revered leader, Grand Ayatollah Ali al-Sistani, who is widely credited with bringing them this far.

Alliance leader Abdel Aziz al-Hakim recently hinted that the Shiites were waiting to take power before dealing with the insurgency. He indicated that a first step would be to identify and purge the security services of any insurgency sympathizers.