CAIRO — Egyptian officials sought to implicate the country’s most influential Islamist group after a powerful car bomb ripped through a police headquarters north of Cairo early Tuesday, killing at least 15 people and wounding more than a hundred others.

The bombing, in the Nile Delta city of Mansoura, was Egypt’s deadliest since militants began a campaign of assassinations and other attacks against the security services in July, when the armed forces ousted Mohamed Morsi as president. Officials have repeatedly accused Mr. Morsi’s Islamist political movement, the Muslim Brotherhood, of orchestrating the attacks, but have provided scant evidence.

Officials said they believed a suicide bomber drove an explosive-laden car through security barriers around 1 a.m. on Tuesday. The explosion rattled windows on the city’s outskirts and led to the partial collapse of several buildings near the police headquarters, trapping officers and civilians in the rubble.

The attack was the second on the headquarters since July and renewed doubts about the military-backed government’s ability to provide security for the public — or its own officers — just weeks before millions of Egyptians are expected to vote in a referendum on a draft constitution. And it seemed certain to strengthen a security crackdown that has been focused primarily on the Brotherhood, but has also lately swept up non-Islamist activists who have been critical of the government’s policies.