Initial findings indicate a suicide car bomb caused Tuesday’s blast in Mansoura, the interior ministry has said in a statement.
Body parts were found inside an exploded car near the site of the attack, the statement added.
A security sweep of the area has been taking place since early Tuesday morning to gather evidence and identify the perpetrators.
The attack in the Nile Delta city killed 15 people and injured 134. The head of the security directorate was among the injured.
The explosion ripped through the security directorate and damaged a number of police vehicles. Nearby buildings, including the city council building, a state-owned theatre and a bank were also damaged.
Prime Minister Hazem El-Beblawi vowed to hunt down the perpetrators, saying the attack aimed to obstruct the roadmap outlining a return to democracy following the outer of president Mohamed Morsi in July.
An army spokesman said the military would continue its crackdown on militant organisations that "toy with Egypt's national security and incite terror among citizens at this critical juncture."
Militant attacks against security targets, mostly in the Sinai Peninsula, have been on the rise since Morsi's ouster. There have been sporadic attacks in the capital and other provinces.
The military has killed and arrested a large number of alleged Islamist militants in a broad counterinsurgency operation in recent months.