People gather near the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Amru Salahuddien)

 

A man stands at the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Amru Salahuddien)

 

Police officers secure the perimeter of the Islamic Museum which was heavily effected by the blast in front of a police headquarters in Cairo, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Amru Salahuddien)

 

An Egyptian woman mourns at the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Amru Salahuddien)

 

People check a damaged car near the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Amru Salahuddien)

 

People gather at the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Amru Salahuddien)

 

Police officers secure the perimeter of the Islamic Museum which was heavily effected by the blast in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Amru Salahuddien)

 

Police officers and investigators secure the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Amru Salahuddien)

 

A woman cries near the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Amru Salahuddien)

 

 A policeman enters the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)

 

Security personnel help an injured at the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)

 

A fire fighter stands at the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)

 

People gather at the damaged museum of Islamic Art opposite to a police headquarters where a car bomb exploded in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)

 

Security personnel stand at the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)

 

Security personnel and an injured man walk out of the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)

 

A man take photos of the damaged facade of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)

 

A policeman stands at the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)

 

Security personnel inspect the blast site in front of a police headquarters in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)

 

 A policeman shouts at the blast site of a police headquarters where a car bomb exploded in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)

 

The damaged Museum of Islamic Art is seen opposite to a police headquarters where a car bomb exploded in Cairo, Egypt, Jan. 24, 2014. At least four people were killed in a powerful explosion which rocked a police headquarters in downtown Cairo early Friday, according to the online edition of Al Ahram newspaper. (Xinhua/Pan Chaoyue)