CAIRO — A powerful explosion believed to have been caused by a car bomb rocked a police headquarters in a Nile Delta city north of Cairo early Tuesday, killing at least 14 people and injuring nearly 100, according to the state news agency and a security official.

The explosion took place in Mansoura, about 75 miles north of Cairo, and it caused parts of the five-floor building to collapse, according to the Middle East News Agency.

No group immediately claimed responsibility for the blast, but the interim government accused the Muslim Brotherhood of orchestrating the attack. The news agency quoted a cabinet spokesman, Sherif Shawki, as saying that the Brotherhood showed its “ugly face as a terrorist organization shedding blood and messing with Egypt’s security.”

The attack came a day after an Islamic militant group, Ansar Beit al-Maqdis, called on police officers and soldiers to desert or face death at the hands of its fighters. Most of those killed Tuesday were police officers.