An explosives expert at the scene of a bombing in Cairo's Nasr City district said the bomb was homemade but concentrated to target as many people as possible.
The expert, shown in a video by Al-Ahram Arabic website, was holding another bomb found and defused at the scene early Thursday.
He stated that the second bomb was planted and aimed at police officers who would arrive at the scene in response to the first bomb.
The public prosecution's initial investigation of the site in Nasr City, and eyewitness testimonies, showed that the bus involved in the incident broke down by chance over the bomb. Most of the passengers had left the vehicle, seeking alternative means of transportation, minutes before the bomb detonated.
The explosion resulted in five injured and no deaths.
The bombings come one day after interim Egyptian government declared the Muslim Brotherhood, from which ousted president Mohamed Morsi hails, a terrorist group.
The interim government reached its decision one day after a deadly explosion rocked the Daqahliya Security Directorate in the Nile Delta city of Mansoura. The bombing, which took place Tuesday morning, resulted in the death of 16, mainly policemen, and the injury of more than 135.
The Muslim Brotherhood denied Tuesday involvement in attacks on government buildings and security personnel.
Ansar Beit Al-Maqdis, an Al-Qaeda-inspired Islamist militant group, claimed responsibility for the Mansoura blast.
Egyptian police announced that no suspects have been arrested yet.