A "significant number" of Nigerian police officers have been arrested over the alleged extra-judicial killing of members of the Boko Haram sect in 2009. Police officials told the BBC more arrests would follow. News agency AP reported that 17 officers were held. Al-Jazeera TV last month showed footage of alleged police killings. Boko Haram attacked a police station in the northern city of Maiduguri, leading to days of clashes and hundreds of deaths - mostly sect members. Police spokesman Yemi Ajyai said the al-Jazeera broadcast had led to the arrests and those detained had been taken to Abuja for questioning. The footage apparently shows police officers telling a group of young men to lie face down and then shooting them at close range. The AFP news agency quoted a police source as saying the the arrests were carried out on the orders of the acting President Goodluck Jonathan. The BBC's Jimeh Saleh in Abuja says the police are often accused of extra-judicial killings but they always deny it and arrests are rare. Boko Haram leader Mohammed Yusuf died in custody after being alive when the army handed him to the police, army officers said. A few hours later, journalists were shown his bullet-ridden body. The police said he had been fatally wounded while trying to evade capture. Source: BBC