MAIDUGURI, Nigeria - Suspected Islamist militants detonated a bomb in a crowded marketplace in northeastern Nigeria killing at least 20 people, witnesses said on Sunday.Nigerian security officials said the attack late on Saturday in the town of Bama in Borno state bore the hallmarks of an attack by the al Qaeda-linked militant group Boko Haram, which is fighting to carve an Islamic state out of northeast Nigeria.Security sources say Boko Haram has killed hundreds, possibly thousands, this year in a campaign of violence that is growing in intensity."I travelled to Bama ...to buy bags of beans. Suddenly, there was a deafening bang at the middle of the market. It was in the late afternoon and commercial activities were at their peak," said Shuaibu Abdulahi, a trader at the market. He estimated the death toll to be as high as 29.Abba Tahir, a bus driver who was offloading passengers at the market, said he counted 20 bodies."People were helping in evacuating the corpses after the confusion had died down. Some people who were injured were taken to the general hospital," Tahir said.There was no claim of responsibility for the attack. The military spokesman for Borno state did not immediately respond to a request for comment.A military crackdown since last May has failed to quell the insurgency, which after four and a half years remains the leading security threat to Africa's top oil producer.