NAIROBI, Kenya — The police intensified their search on Thursday for the plotters of the deadly assault on a Nairobi hotel-shopping complex, as Kenyan news services said at least 11 people had been arrested, including a suspected financier.

The assault, which began Tuesday afternoon and lasted more than 18 hours, killed at least 21 people, including a police officer. Fifteen of the victims were Kenyan, one was American, one was British and the others were of undetermined African nationality, the authorities said.

Five assailants also were killed and 700 people were evacuated from the complex in one of the Kenyan capital’s most secure areas.