Egyptian security officials have blamed Islamist militants for the shooting death of an Interior Ministry lieutenant colonel, the highest-profile killing in Cairo since the overthrow of Islamist President Mohamed Morsi in July.Mohamed Mabruk was killed outside his home in Cairo's Nasr City district on Sunday, three days after a three-month curfew and state of emergency were lifted.Mabruk, who was shot by masked men, was in charge of monitoring Morsi's Muslim Brotherhood movement in the Interior Ministry's National Security division."Our investigations point to Islamic jihadists carrying out this assassination with political motives," a security official told Reuters. "They were taking revenge on Mabruk because he took care of a very high profile file."The army-backed government accuses the Brotherhood of carrying out militant attacks. It makes no distinction between the Brotherhood and militants in Sinai, who attack security forces almost daily.The Brotherhood, which won every election since strongman Hosni Mubarak was toppled in February 2011, says it is a peaceful movement.The killing is bound to raise fears that an Islamist insurgency could take hold beyond the Sinai Peninsula, where militants have escalated attacks since the army overthrew Morsi following mass protests against his rule.Political turmoil and attacks by Al-Qaeda inspired Islamists in the Sinai near Israel have hurt investment and tourism in Egypt, one of the United States’ most important Arab allies. Islamists waged a full-blown insurgency in Egypt in the 1990s which was eventually crushed by then president Mubarak.Security sources said Mabruk was shot seven times. There has been no immediate claim of responsibility for the attack.Mabruk was involved in sensitive work. He helped to uncover the identity of a former military officer who staged a failed suicide bombing attack on the interior minister in Cairo in September.He was also in charge of revealing the location of Muslim Brotherhood members who attempted to assist leaders of the movement flee the country after the army takeover, the security sources said.The Brotherhood has seen hundreds of its members killed in a security crackdown and thousands arrested. Morsi and 14 other senior Brotherhood figures are on trial for inciting violence.The army says a political road map will lead to free and fair elections in Egypt, where the popular uprising which toppled Mubarak raised hopes of democracy after decades of authoritarian rule.