Baghdad ? A female suicide bomber struck a tent filled with women and children resting during a pilgrimage south of Baghdad on Friday, killing 40 people and wounding about 80 in the deadliest of three straight days of attacks against Shiite worshippers.

The grisly assault, which also appeared to be the deadliest in Iraq this year, demonstrates the determination of some extremists to reignite sectarian warfare. It also underscores how fragile security remains here, even as the U.S. turns over more responsibility to the Iraqis.

Witnesses said many of the injured were hurt in a stampede as terrified survivors — most of them poor Shiites exhausted after days of walking — scrambled away from the tent in terror.

They left behind piles of clothing, small rugs and toddlers’ strollers, Associated Press Television News video showed. A dismembered leg believed to have been the bomber’s lay wrapped in an abaya in a cardboard box.

“It was a horrific scene with dead and screaming injured people on the ground,” said Sadiya Kadom, 40, a Baghdad resident who was near the tent when the blast occurred.

No group claimed responsibility. But suicide bombings against Shiite civilians are the signature attack of al-Qaida in Iraq, which U.S. commanders say has been severely weakened but not defeated.