By Imtiaz Shah

KARACHI, Oct 26 (Reuters) - Pakistani police released a photograph of the reconstructed face of a second man suspected to have been involved in a suicide attack on former Prime Minister Benazir Bhutto that killed 139 people, police said on Friday.

A photograph of the head of the first suspect was released a day after the attack on Bhutto's procession in Karachi on Oct 19, following her return from eight years of self-imposed exile.

Police are unsure whether there was one or two suicide bombers. Initially, they had thought the first blast was caused by a grenade.

A week after the devastating main blast neither of the suspects have been identified from the photographs circulated.

Typically, the upward force of the blast from explosives packed in a suicide vest causes a bomber's head to be blown off.

The latest photo released late on Thursday showed a badly damaged head of a man with a trimmed beard and moustache. The face was scraped. The eyes and forehead appeared badly damaged.

Police have detained some people for questioning, but it is unclear whether this stemmed from evidence or was part of a sweep in the hope of unearthing a lead.

"The police have detained some people. I cannot comment on the number of people detained or their affiliation," Waseem Akhtar, an adviser to the provincial government in Sindh, said on Friday.

The investigation has been mired in controversy.

On Thursday, a new chief investigating officer began familiarising himself with the findings so far, after the first officer withdrew following complaints from Bhutto that he had been present during the torture of her husband while he was in custody in 2000.

NO OUTSIDE HELP WANTED

Bhutto's supporters have been highly critical of the way the crime scene was handled after the attack, and their leader has asked for foreign experts to be brought in to help the investigation -- which President Pervez Musharraf has refused.

Government officials have said the attack was mounted by Islamist militants from tribal areas on the Afghan border that have become strongholds for al Qaeda and Taliban fighters.

Bhutto has said she believes senior members of the Pakistani establishment were plotting against her, adding she did not believe Musharraf was involved at any level.

Her statements have provoked angry reactions from Musharraf's allies in the ruling Pakistan Muslim League, raising doubts about whether the PML could ever agree to an alliance with Bhutto's Pakistan People's Party (PPP).

Musharraf granted an amnesty earlier this month that allowed Bhutto to return free of fear of prosecution for any of the corruption charges that have been hanging over her since the 1990s.

There is strong speculation the concession would lead to an agreement to share power after national elections due in January.

The United States is believed to be quietly encouraging Bhutto and Musharraf to work together in the hope of providing stability in nuclear-armed Pakistan, as they are both seen as friendly to the West.

But Bhutto, however, says such talk is premature.

For one thing, the Supreme Court has still to decide whether parliament's re-election of Musharraf on Oct. 6 should stand because he contested while still army chief.

Musharraf has said he will quit the army and become a civilian leader if his re-election is ratified.

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