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In Pakistan, hundreds attend mass funeral for 45 minority Shiites killed in Karachi bus attack

  • People light candles to protest and show solidarity with the victims of a bus attack in Karachi, Pakistan, Wednesday, May 13, 2015. Gunmen stormed a bus in southern Pakistan and ordered its Shiite Muslim passengers to bow their heads before shooting them, killing dozens of people in the latest attack targeting religious minority, officials said. (AP Photo/Shakil Adil)

    People light candles to protest and show solidarity with the victims of a bus attack in Karachi, Pakistan, Wednesday, May 13, 2015. Gunmen stormed a bus in southern Pakistan and ordered its Shiite Muslim passengers to bow their heads before shooting them, killing dozens of people in the latest attack targeting religious minority, officials said. (AP Photo/Shakil Adil)  (The Associated Press)

  • People pray for the victims of a bus attack, in Karachi, Pakistan, Wednesday, May 13, 2015. U.N. Secretary-General Ban Ki-moon is urging Pakistan's government to take "swift measures" to protect religious minorities, including Shiite Muslims and Christians, following a number of attacks. (AP Photo/Shakil Adil)

    People pray for the victims of a bus attack, in Karachi, Pakistan, Wednesday, May 13, 2015. U.N. Secretary-General Ban Ki-moon is urging Pakistan's government to take "swift measures" to protect religious minorities, including Shiite Muslims and Christians, following a number of attacks. (AP Photo/Shakil Adil)  (The Associated Press)

  • Women mourn as they visit a local hospital following an attack on a bus in Karachi, Pakistan, Wednesday, May 13, 2015. Gunmen killed dozens of people on Wednesday aboard a bus in southern Pakistan bound for a Shiite community center, in the latest attack targeting the religious minority, police said. (AP Photo/Shakil Adil)

    Women mourn as they visit a local hospital following an attack on a bus in Karachi, Pakistan, Wednesday, May 13, 2015. Gunmen killed dozens of people on Wednesday aboard a bus in southern Pakistan bound for a Shiite community center, in the latest attack targeting the religious minority, police said. (AP Photo/Shakil Adil)  (The Associated Press)

Amid tight security, hundreds of Pakistanis have paid their respects at a mass funeral for 45 minority Shiites killed in an attack the day before on a bus in the southern city of Karachi.

Pakistan is observing a day of mourning Thursday over the killings. State-run television is broadcasting live footage showing mourners attending the last rituals for the victims. Gunmen had stormed the bus with the Shiites on Wednesday, ordered them to bow their heads and then shot them dead.

It's unclear who was behind the assault. Both a Pakistani Taliban splinter group and an Islamic State affiliate claimed responsibility for it. Pakistani Sunni militant groups view Shiites as apostates and have targeted them in the past.

Prime Minister Nawaz Sharif had rushed to Karachi and ordered an investigation.