By Paul Reynolds BBC News Online World Affairs correspondent

The antics of Zacarias Moussaoui in a US Federal Court in Virginia have presented the authorities with a dilemma.

The worst result for the authorities would be for Moussaoui to get off on a legal point

Do they seek the death penalty for the only man to be charged over the 11 September attacks?

Or do they try to strike a deal with him under which he talks, possibly about other plots, and gets a long prison sentence, probably life, instead?

On the one hand, the Justice Department will be satisfied that his outburst has confirmed some at least of the allegations they have made against him.

On the other, the needs of intelligence might be paramount.

'Existing conspiracy'

Plan A was to seek the death penalty.

The 11 September attacks claimed more than 3,000 lives

The way for this was cleared when the grand jury - the body drawn from ordinary citizens which actually brings in an indictment - said that there were aggravating circumstances in the alleged conspiracy.

The grand jury said that the crimes were "especially heinous, cruel and depraved". Without such "specials findings", as they are known, the death penalty cannot be sought in a federal case.

Moussaoui, however, has thrown a spanner into the works by leaping up in court, declaring his guilt and his membership in al-Qaeda.

But he also said - and this was probably the most interesting part of his sudden intervention - that he had "certain knowledge about the 11 September" and that "I have many, many information to give to the American people about an existing conspiracy".

Was he hinting at a plea bargain under which he might talk in exchange for not facing the death penalty?

Or is he planning to reveal all to a jury which would have to decide on a death sentence in the hope that they will send him to prison instead?

What this "existing conspiracy" might be nobody knows, though it has been reported that some investigators think that Moussaoui might not have been part of the 11 September group but planning something else.

Legal confusion

The temptation exists for the authorities to develop a Plan B - to take him up on his implied offer.

I know which group, who participated, when it was decided - I have many information

Zacarias Moussaoui

It might yield tremendously valuable information, not only about what really happened before 11 September but about other plans al-Qaeda may have had - or might still have.

The immediate problem though is the legal confusion Moussaoui has created.

The US District Judge, Leonie Brinkema, did not accept his "plea" of guilty and told him to consider his position. This was only prudent. Moussaoui refuses to have a lawyer.

The court has to avoid opening up any avenue of appeal. His "guilty" cry might not have actually applied to all six counts on the indictment, for a start.

The worst result for the authorities would be for Moussaoui to get off on a legal point.

But another problem is that Moussaoui himself does not apparently know how the plea bargaining system works.

It depends on a clear understanding by both sides about the final outcome, not dramatic moments in open court. If he wants a bargain, he will have to negotiate for one.

And above all, he will have to talk.