The US military on Saturday blamed the bombing of a pet market in Baghdad the day before on Iranian-backed Shi'ite militants, raising concerns that Shi'ite extremist activity could escalate and jeopardize waning violence in Iraq. The bomb that was hidden in a box of small birds exploded Friday morning as Iraqis were strolling past animal stalls and bird cages at Baghdad's al-Ghazl market. The market had recently re-emerged as a popular venue as security has increased in the capital. US military spokesman Rear Adm. Gregory Smith said the bomb was packed with ball-bearings to maximize casualties. "In raids overnight, Iraqi and coalition forces were able to identify and detain four members of a militia extremist group we assess as responsible for this horrific act of indiscriminate violence," he said at a news conference. "Based on subsequent confessions, forensics and other intelligence, the bombing was the work of an Iranian-backed special cell operating here in Baghdad."