CAIRO - Egypt's most dangerous militant group on Friday posted its first video message since pledging allegiance to Islamic State, with footage purporting to show that the group was behind one of the most deadly attacks on Egyptian security forces in years.The nearly 30-minute video, whose authenticity could not immediately be verified, was posted on the Twitter feed claiming to represent Sinai-based Ansar Bayt al-Maqdis.The group this week pledged loyalty to the al Qaeda offshoot now facing US air strikes in Syria and Iraq. It later changed its name to Sinai Province on the Twitter feed, suggesting loyalty to the self-declared Muslim caliphate.The video shows a man threatening supporters of President Abdel Fattah al-Sisi. Text at the bottom of the video describes him as the suicide bomber who attacked the Karam al-Kawadis military site.The video then cuts to footage of a large explosion in the desert. Gunmen are shown entering an area littered with dead men, some in military fatigues. Another image shows fighters climbing on top of a tank and raising the black flag of the Islamic State.A suicide bombing and a subsequent gun attack at Karam al-Kawadis in northern Sinai on Oct. 24 killed more than 30 security personnel and prompted the government to declare a three-month state of emergency in the area.