Several Islamic groups claimed responsibility for the killing of Mumcu, a relentless campaigner against Islamic radicalism. The most likely candidate was The Islamic Liberation Organisation, which has killed secularist authors before.
'The caller said it was to punish him,' said a colleague at Mumcu's newspaper, the respected left-wing daily Cumhuriyet, where staff were in shock and tearful mourning after the 50-year-old veteran's death near his house in Ankara.
Turkish leaders may have been at the receiving end of his merciless pen but they all condemned the killing, including those of the the pro-Islamic Welfare Party.
State television devoted the first 40 minutes of its main evening news to the murder, clearly hinting, despite a recent official thaw between Ankara and Tehran, that responsibility might lie with Islamic revolutionary radicals in Iran.
Mumcu's prolific output of books and his daily front-page column made him the most important left-wing commentator in the country.
His controversial - but meticulously researched - articles never failed to name names.
Most recently, he had been working on a book that claimed to link Turkey's Kurdish guerrillas to the secret police, who are supposed to be fighting them. The scope of his targets was wide. He wrote about not just Kurdish guerrillas, Islamic extremists and Turkish police methods - he was arrested after the 1971 coup - but also Turkey's powerful drug gangs, foreign intelligence agencies and the need for Turkey to distance itself from Nato.