However Islamabad has denied the group were operating in the country, which has been wracked by Al-Qaeda and Taliban-linked violence for more than a decade.
Islamic State militants were behind a brutal attack that killed 45 Shiites in Karachi in May, a Pakistani official said on Tuesday, despite the government's long-held stance that the jihadist group does not have a presence in the country.
Gunmen stormed a bus in Karachi in May, slaughtering members of the Ismail minority community in one of the deadliest incidents in Pakistan this year. ISIS swiftly claimed responsibility, marking the first time the militants, who have seized a large swathe of Syria and Iraq and declared a "caliphate", said they were behind an attack in Pakistan.
However Islamabad has denied the group were operating in the country, which has been wracked by Al-Qaeda and Taliban-linked violence for more than a decade. In June, intelligence and police officials investigating the Karachi attack had suggested IS might be linked through another militant group, Lashkar-e-Jhangvi, the bloodiest and most ruthless anti-Shiite outfit in Pakistan.
Investigators said the attack may have been carried out by LeJ as it seeks to expand its international influence -- and get access to IS's rich funding. But on Tuesday Rehman Malik, Pakistan's former interior minister and the head of a Senate committee on the interior, said IS was "100 percent" behind the Karachi attack, citing testimony from the Sindh provincial police chief to his committee.
"IS was 100 percent involved in it, the attack was planned from Syria and the attackers were receiving orders from a Pakistani member of IS presently in Syria," Malik told AFP after the Monday meeting where the Sindh police chief testified.
"So far 14 people belonging to IS have been arrested in Pakistan." When contacted, a spokesman for the interior ministry said he has not seen the testimony so he was not in a position to comment. Malik said Abdul Aziz, the alleged mastermind of the attack, is from the city of Jhelum in Punjab province, and lived in interior Sindh for some time before moving to Syria.
Security analyst Imtiaz Gul said Islamabad's claim that IS was not operating in the country was "unrealistic". "There are people from Tehreek-e-Taliban Pakistan (TTP) and other militant groups who fight in Pakistan and Afghanistan for money and IS is an attraction for these people," Gul said. He said LeJ was also inclined towards IS, but he added that these people fight for money and have no religious agenda.
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