A bomb hidden in a cart of vegetables ripped through a crowded market in Baghdad's Sadr City on Wednesday, killing at least 56 people, Iraqi officials said, just days after the US military closed its main base in the Shiite district. It was the third deadliest attack this year, coming less than a week before a deadline for US combat troops to leave Iraqi cities under a new security pact. The 7 p.m. blast seemed to be timed to maximize casualties by striking shoppers buying food for their evening meal. The explosives were loaded on a motorized pushcart and shrapnel was blown more than 600 yards away, a police officer said. Police and hospital officials said more than 100 people also were wounded. They spoke on condition of anonymity because they were not authorized to release the information. The explosion also set some shops ablaze in the market. US and Iraqi officials have warned that violence would likely increase before the June 30 deadline for most American troops to pull back from urban areas - the first stage of a full withdrawal by the end of 2011. Brig. Gen. Steve Lanza, a US military spokesman, said earlier Wednesday that the high-profile bombings were an effort to rekindle sectarian violence that pushed the country to the brink of civil war. As part of the withdrawal, American commanders on June 20 turned over control of a key base on the edge of Sadr City. The base served as the hub for all operations in the sprawling slum, which was a militia stronghold where fierce clashes took place before a US-backed government crackdown and a cease-fire. Four days earlier, a truck bombing killed 82 people in a mainly Shiite town near the northern city of Kirkuk, which was the deadliest bombing so far this year. Back-to-back suicide bombings by female attackers also killed 71 people outside a Shiite shrine in Baghdad on April 24.