PARIS — The police organized an enormous manhunt across the Paris region on Wednesday for three suspects they said were involved in a brazen and methodical midday slaughter at a satirical newspaper that had lampooned Islam.

The terrorist attack by masked gunmen on the newspaper, Charlie Hebdo, left 12 people dead — including the top editor, prominent cartoonists and police officers — and was among the deadliest in postwar France. The killers escaped, traumatizing the city and sending shock waves through Europe and beyond.

Officials said late Wednesday that two of the suspects were brothers. They were identified as Said and Chérif Kouachi, 34 and 32. The third suspect is Hamyd Mourad, 18. News reports said the brothers, known to intelligence services, had been born in Paris, raising the prospect that homegrown Muslim extremists were responsible.