The case has also given a severe jolt to the government of West Bengal state, of which Calcutta is the capital.

Police are accused of involvement in the death, and of aiding Rahman's in-laws, a rich Hindu family who had opposed his marriage to Priyanka Todi, a student.

The federal Central Bureau of Investigation has now taken up the case at the urging of the state's High Court, and said Ms Todi's father, the well-known industrialist Ashok Todi, is being investigated for involvement in the suspected murder.

'We don't have faith in [West Bengal] state and its agencies. We believe the federal agency is more impartial and is capable of bringing my brother's murderers to book,' said the dead man's brother, Rukbanur Rahman.

From a Muslim ghetto, but having studied at a prestigious missionary college, Rizwanur Rahman became a teacher at a popular multimedia institute where he met Ms Todi. They fell in love and got married, despite strong opposition from Mr Todi.

In the sort of love story normally reserved for Bollywood, Ms Todi left her rich parents and life of luxury to live in a one-room home with her new husband in August.

However, according to a letter the couple sent to local police and a human rights group, Ms Todi's father soon began pressuring the couple to separate and insisted his daughter return to him.

The letter also said that Mr Todi, who has strong connections to the Communist Party leadership, then threatened Rahman with 'dire consequences' if he did not let his daughter return to her parents.

She eventually agreed to visit her family for a week, Rahman said in a second letter to human rights groups, but she never returned to the couple's home. He said police had threatened to charge him with abducting his bride.

'More than a week has gone by and my wife has not returned to me. I am not even being allowed to talk to my wife over phone ... I am convinced she is being detained in her father's custody against her wishes. Please help me get back my wife,' he wrote to the Association for the Protection of Democratic Rights.

He also alleged Mr Todi sought help from senior police officers to take his daughter back.

The rights group believes police took a partisan role and separated the legally married adults against the couple's wishes.

'Police forced Rizwan to sign an illegal bond which was personally written by a senior police officer. The bond said Priyanka would be returned to Rizwan's house after one week,' said Sujato Bhadra, of the rights association. 'When Priyanka did not return to Rizwan after one week, police did not take a proactive role against Ashok Todi's family on charges of breaching the bond. Writing such a bond in the situation was completely illegal. It is crystal clear, police acted in favour of Ashok Todi.'

Ms Todi was this week interviewed by CBI officers, who told the media local police had been pressuring the couple. She said she believed her husband committed suicide, however, and said her father was blameless in the case.

Since the discovery of his body on September 21, demonstrations, candle-light vigils and online petitions have called for the case to be pursued, with many convinced Rahman was killed because he was a Muslim. The case has strong political implications, with Muslims and leftist intellectuals criticising the Communist government, which had previously been able to count on their support.

'The government took a long time to act in this case,' said Manju Majumder, secretary of the Communist Party of India in West Bengal.

Rahman's relatives believe the involvement of the CBI brings them a step closer to justice. However, his mother, Kiswar Jahan, said: 'No court or government can ever compensate this loss in my life.'