By the BBC's Owen Bennett-Jones

The trial of Omar Sheikh presents the Pakistani Government with formidable legal and political difficulties.

He has been charged with kidnapping, murder and terrorism.

But state prosecutors openly acknowledge that while they have received a video tape showing Daniel Pearl's death, they are hampered by their failure to locate either his body or the weapon used to kill him.

Investigators are still looking for Pearl's body

Eleven people have been charged with involvement in the murder but only four of them are in custody.

Prosecutors also face the problem that the complainant in the case, Pearl's wife, Mariane, is not in the country.

Statement

Since Mariane Pearl is heavily pregnant she might be unable to appear in the Karachi court and it is not yet clear that the judge will accept a written statement from her.

Under Pakistan's anti-terrorist laws, the trial of Ahmed Omar Saeed Sheikh should last no more than seven days.

In practice, it could drag on for months.

The state has prepared a long list of witnesses who will give evidence against Omar Sheikh.

They include FBI agents and a taxi driver who saw Daniel Pearl get into a car with Omar Sheikh.

But so far the police have been unable to find anyone who admits seeing the murder.

Intelligence links

The government in Pakistan faces political as well as legal problems.

Officials insist that the country's main intelligence agency, the ISI, had nothing to do with the kidnapping of Daniel Pearl.

But privately they acknowledge that the ISI could have had dealings with Omar Sheikh in the past because of his long-standing interest and involvement in the Kashmir issue.

Omar Sheikh is defending himself

Observers believe the Pakistani government is reluctant to interrogate Omar Sheikh too thoroughly for fear that he could reveal his past links with Pakistan's intelligence community.

Omar Sheikh, who says he will conduct his own defence, is likely to use the trial as an opportunity to bolster his status as a leading Islamic radical.

He has already denied the charges against him and used the court to issue threats against America.

Extradition

Pakistan's concerns about the possible revelations Omar Sheikh could make might also be affecting the government's response to an American request for his extradition.

Tight security surrounds the trial

Pakistani officials now say they want to complete the trial of Omar Sheikh on Pakistani soil before making a final decision on extradition.

General Musharraf knows that the trial of Omar Sheikh will attract considerable international attention.

A conviction could help underpin Musharraf's image as a leader determined to control Pakistan's Islamic extremists.

But the lack of evidence against Omar Sheikh and the risk that any full confession from him could prove embarrassing means the outcome of the trial is far from certain.