Suspected members of the Boko Haram militant group have shot dead 15 people in an attack on a local market in northeastern Nigeria, residents have said.The attack took place in the town of Gajiran in Borno state on Thursday, AFP reported. Gajiran residents said the gunmen pretended to be traders attending the market. "Some of them came aboard trucks while others came on foot to beat the security checks at the entrance of the town," resident Ibrahim Bulama said. They then "blended among traders conducting business," before opening fire in the market, killing 15 people, he added. Borno state is considered Boko Haram's stronghold and has seen a spate of similar attacks on locals in recent weeks. On May 15, the Nigerian army launched an offensive against Boko Haram, a day after President Goodluck Jonathan declared a state of emergency in the three northeastern states of Borno, Yobe, and Adamawa. Boko Haram -- whose name means “Western education is forbidden” -- says its goal is to overthrow the Nigerian government. The group has claimed responsibility for a number of deadly gun and bomb attacks in various parts of Nigeria since 2009. Over the past four years, violence in the north of Africa’s most populous country has claimed the lives of 3,600 people, including killings by the security forces.