At least three people were killed on Sunday when an explosion ripped through Punjab Home Minister Shuja Khanzada's office in Pakistan, authorities said.The incident took place in Attock city, leaving 20-25 people trapped under the rubble as the entire building collapsed, Dawn online reported.The minister's staff confirmed that Khanzada was in the office at the time of the blast and was injured.The nature of the explosion was not known although some witnesses claimed that a suicide bomber carried out the attack.