CAIRO — Homemade bombs built from pressure cookers, a version of which was used in the Boston Marathon bombings, have been a frequent weapon of militants in Afghanistan, India and Pakistan. Al-Qaeda’s branch in Yemen once published an online manual on how to make one, urging “lone jihadis” to act on their own to carry out attacks. President Barack Obama underlined Tuesday that investigators do not know if the twin bombing the day before that killed three people and wounded more than 170 was carried out by an international organization, a domestic group or a “malevolent individual.” There has been...