LAHORE, PAKISTAN, March 19 (CNA) - A Catholic man who died in a Pakistani prison while serving a life sentence for blasphemy might have been murdered by extremists, his supporters said.

While doctors said Qamar David, 55, died of a heart attack, the Catholic bishops' National Commission for Justice and Peace suggested he might have "fallen prey to an active hate campaign going on in the country on this issue" at the hands of "extremist groups."

Church leaders and human rights groups have called for an independent investigation of his death.

David, a wealthy businessman, was found dead on March 15. Both he and his lawyer received threats of violence regularly throughout the course of his time in prison and at nearly every court hearing in his case, a representative of Christian Solidarity Worldwide told Vatican Radio.

Fr. Mario Rodriguez, the national director of the Pontifical Mission Societies, said that the death "shocked us." The Catholic Church is calling for "clarity" and an official report of the facts.

"We have contacted the prison authorities and spoken with the detainee who shared David's cell. He told us that David was fine but he was very afraid. Given that he was accused of blasphemy, he was often badly beaten. The story of a heart attack is unconvincing," he told Fides news agency.

Haroon Barkat Masih, director of the human rights and minority advocacy Masihi Foundation, said he was convinced David died because of beatings carried out by staff and other inmates.

"The police and the hospital board want to cover up the true cause of death," he said.

David was arrested in 2006 for possessing a phone that was used to send derogatory messages insulting the prophet Muhammad. A Muslim who was also accused in the case was acquitted for lack of evidence. David, a Catholic man, was given life imprisonment and fined $1,183 in 2010 under the blasphemy laws, according to UCA News.

His lawyer, Pervez Chaudry, maintains that the allegations were spurious and triggered by a business rivalry, UCA News reports. He blamed the conviction on pressure from local religious clerics and their supporters.

Auxiliary Bishop Sebastian Shah of Lahore presided over the March 17 funeral at St. Joseph Church in Lahore, David's native city. Fr. Andrew Nisari, the diocese's vicar general, asked the congregation of more than 200 to be undeterred in their faith.

David's death was "another historic chapter" in the struggle against the "fatal logic" of blasphemy laws, the priest said in his address. Persecution and discrimination, especially in employment, has frustrated Christian youth, he added.

The controversy over Pakistan's blasphemy law has contributed to the assassinations of Punjab governor Salman Taseer and Pakistan religious minorities minister Shahbaz Bhatti, both opponents of the law.

At present, Christian mother Asia Bibi is imprisoned and faces a death sentence. She told Fides news agency in a recent interview that she worries every moment may be her last.

"Each time the door of my cell is opened, my heart beats a thousand miles an hour. I am in the hands of God. I do not know what will happen to me. In prison anyone can be judge and killer," she said.