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Auj, 54, had earlier complained to the police about death threats he began receiving after delivering a speech in the United States in 2012, his colleagues and the police said.

Nasir Lodhi, a senior police official, said Auj told the police that four professors at the University of Karachi had accused him of blasphemy for comments he made during that speech. Lodhi said he could not say where the speech was made, or the nature of the offending comments.

Auj lodged a criminal complaint against the four professors, who were later arrested by the police. One of them, Abdul Rasheed, had previously held Auj’s position as dean of Islamic studies at the university. The four men face trial but are free on bail, the police said.

Around the same time, a religious seminary in Karachi issued a fatwa against Auj, accusing him of blasphemy and calling for his death.

Pir Muhammad Shah, a senior police official, said the four professors were being questioned again after Auj’s killing. “At this stage, it is premature to say anything about the killing of Auj.”

Blasphemy is punishable by death under Pakistani law, and accusations of blasphemy have inspired a rising tide of vigilante killings in recent years that are seen as a sign of growing intolerance in the country.

Human rights groups say the laws are frequently abused in pursuit of personal or professional grudges.

Auj, who was considered a progressive liberal in his field, had written 15 books about Islam and was a regular participant in television debates about religious issues, according to a profile on the University of Karachi website. Last month, the government awarded him a presidential medal of distinction for his contribution in the fields of education and research.

The Karachi police chief, Ghulam Qadir Thebo, announced a reward of 2 million rupees (about $20,000) for information leading to the arrest of Auj’s killers.

His students mounted a protest outside Karachi University. On campus, some teachers said they would indefinitely boycott their classes.

“The government has failed to protect our teachers,” said Ahmad Ali Shah, a student at the Islamic studies department, during the protest.