Cairo. At least four people have been injured in an explosion near the military intelligence building in Sharqiya, in Egypt’s Nile Delta, the third bombing on the mainland in less than a week.Three men left a car laden with 20kg of explosives by the entrance to the building on Sunday morning, and detonated it from a distance with a remote control, according to an Egyptian army spokesperson. The military is conducting a search of the the area for the assailants. Colonel Ahmed Ali, a spokesman for the army, said the intelligence building was partly damaged by the blast. There was no immediate claim of responsibility.Attacks on police and military installations have become a regular occurrence in Egypt.On Tuesday, a powerful car bomb exploded near a police station in Mansoura, another city in the Delta, killing 14 people and injuring more than 150 others. Ansar Bayt al-Maqdis, a Sinai-based group, claimed responsibility for the bombing in a statement posted online.But the army-backed interim cabinet nonetheless blamed the Muslim Brotherhood, and the next day designated the Brotherhood as a "terrorist organisation.”Hundreds of members have been arrested over the past few days for staging protests, and there were reports in local newspapers on Sunday that some of the group’s remaining leadership had fled the country.On Thursday, a homemade bomb exploded near a bus in Cairo, injuring five passengers. Police said they defused two other bombs planted in the same area. It was one of the first attacks in Cairo that targeted civilians.Ansar Bayt al-Maqdis has staged hundreds of attacks against police and soldiers on the Sinai peninsula, but it has recently carried out several high-profile bombings on the mainland.The group claimed credit for a September assassination attempt on the interior minister in Cairo.