Anti-terrorism Pakistani experts are investigating the suicide attack on the United Nations office in Islamabad, which has killed an international and four local employees, Pakistani Interior Minister Rehman Malik said Tuesday. A bomber in police uniform struck the highly guarded World Food Programme (WFP) office, raising questions about the ability of the security system in place in the Pakistani capital. Malik said that a Joint Investigation Team (JIT) has started probe into the attack and the preliminary report is likely to be ready in a couple of days. CCTV footage shows that the bomber, wearing black uniform of police, entering the WFP building. He detonated his bomb in the lobby of the building. An Iraqi national and two Pakistani UN staff died at the scene and two later died in hospital. The UN has temporarily closed all offices in Pakistan, according to the UN head of mission in the country. 'It is a complicated matter and the investigation may take some time,' the Interior Minister told reporters after visiting the attack's injured at Islamabad's main hospital. He said the attack is the result of negligence of the security guards and all private security guards have been taken into custody for questioning. He said data of all employees at the WFP office are being collected as part of the investigation process. Malik said the law enforcement agencies have been told that identity cards of every person in police uniform and in government cars should be checked at all check posts. The minister again blamed the Pakistani Taliban for the WFP office attack and said the 'group in a meeting has decided more attacks'. He said there had been no specific intelligence about the attack on the WFP office. 