It's been three years since I covered a 9/11 pre-trial hearing at Guantanamo.

By now, I thought I'd be in the middle of covering the trials of the five men who have been accused of planning the 2001 attacks. Certainly, that was what prosecutors and defence lawyers expected – they thought the trials were going to start in 2015.

But now, four weeks before the start of 2017, it is thought that the trials may not start until 2019.

The pace of this military commission has mystified some observers and angered many relatives of those killed in the attacks. They wonder what is taking so long? Why is so much time being spent on the health of the defendants, their legal rights and other, seemingly arcane, matters?

There are several reasons.

This is a death penalty case, and defence lawyers say they have an obligation to represent their clients vigorously. That means trying to obtain every piece of evidence that could prevent a conviction, or barring that, keep the defendants out of the execution chamber.

When it comes to witnesses, both sides have to travel internationally to interview them – that also takes time.

Moreover, the case involves classified information, and there has already been considerable debate about how this evidence is gathered and shared with opposing counsel. Decisions are thus very slow in coming.

The defendants are being tried in a military commission – a venue whose legitimacy has been challenged.

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While the defence and human rights groups have questioned why this case isn't being tried in a federal court, prosecutors say it is a proper venue because the suspects were caught on the battlefield and because the commission's process has been reformed to ensure a fair trial.

Finally, even if the defendants are convicted, we should expect years of appeals. Chief among the potential grounds for appeal is the torture many of the detainees were subjcted to while in CIA custody.

Patience is required to cover this story. But, it's going to take many more years for the relatives of those killed on September 11 to see if the defendants will be held accountable.