The tragic assassination of Pakistan's first woman Prime Minister Benazir Bhutto still remains a mystery as her party is set to observe the 6th anniversary of her death on Friday.

No group has yet claimed responsibility for the murder and the trial of some suspects is still underway.

Benazir Bhutto, a charismatic leader who served Pakistan twice as prime minister, was killed in a suicide attack shortly after she addressed a huge election rally in the garrison city of Rawalpindi on Dec 27, 2007.

The then government of Pervez Musharraf was quick to blame Pakistani Taliban founder Baitullah Mehsud and al-Qaeda for masterminding the attack. Bhutto had been strongly critical of the Taliban which until now has continued its suicide attacks in some parts of Pakistan.

Security officials had released audio tape of two persons, whom they described as Taliban operatives, who greeted each other on the Benazir's assassination.

The Taliban had, however, denied involvement and blamed the security establishment for the assassination.

The leadership the Pakistan Peoples' Party (PPP), including former President Asif Ali Zardari, the spouse of Benazir Bhutto, had also pointed fingers at the Taliban on some occasions. But the mystery has not yet been resolved as to who had masterminded the plot to eliminate the world-recognized woman leader.

In view of demands for investigation from foreign teams, the Musharraf government had sought help from the British Scotland Yard, whose team had just probed the circumstances that led to the attack and causes of Benazir Bhutto's death.

Later a United Nations' team carried out a thorough investigation at the request of the then government of Pakistan. The UN inquiry reported in April 2010 that the security arrangements by the federal and the local authorities to protect Benazir Bhutto were "fatally insufficient and ineffective" and subsequent investigations into her death were prejudiced and were more of a whitewash.

But the UN Commission of Inquiry, appointed by Secretary- General Ban Ki-moon, has reached no definitive conclusion as to who the organisers and sponsors of the attack were.

Investigators have formally included former President Pervez Musharraf in Bhutto's murder case since he was the prime minister at the time of the attack.

Investigators have officially blamed Musharraf's government for failing to provide enough security to her.

An anti-terrorism court, hearing the Benazir Bhutto assassination case, started formal trial of Pervez Musharraf in connection with the assassination of Benazir Bhutto in August. Musharraf has dismissed all charges as politically-motivated.

The court, hearing the case inside a prison over security threats, has already indicted five alleged members of militant groups, who were identified as Hasnain Gul, Rafaqat Hussain, Sher Zaman, Aitzaz Shah and Abdul Rasheed for helping the attackers.

Most of them had denied any involvement. Before her assassination, Benazir had been under threats from the militants and extremist groups for her firm stance against violent extremism.

Thousands of supporters and leaders of the PPP will gather at her mausoleum at Garhi Khuda Bakhsh, a town in southwestern Sindh Province, on Friday to pay homage to their leader on her 6th death anniversary.

It is expected that they would press for the government of Nawaz Sharif to expedite the solution of the murder of Bhutto to put a closure to the heinous crime perpetrated against the daughter of the founder of Pakistan.

It is not only Bhutto's party but the whole Pakistani nation anxiously wants to see the conspirators exposed as they have deprived the country of an internationally-recognised leader.

It is now a test case for all investigators to bring to justice the real killer or killers of Benazir, the first Muslim woman to become prime minister of the country.