The first and most obvious - and the one that French officials have been trying hard not to take for granted - implicates Algerian Islamic fundamentalists; the second, Bosnian Serbs. A third theory casts suspicion on Palestinians opposed to the current peace talks with Israel.
Some cast the net wider, to include supporters of a French-Moroccan terrorist, Tarek Falah, who is about to be extradited to France from Germany, to face charges in connection with the killing of two Spanish tourists in Marrakesh a year ago; supporters of the international terrorist Carlos, who is in a French prison awaiting trial; people linked to the Japanese Aum sect; extremists opposed to France's proposed resumption of nuclear tests, or - the most prosaic suggestion - Corsican separatists. The only conclusion to be drawn from this list is that France currently boast a great many potential enemies.
But it is the first three theories and variations on them that are seen as most likely. The arguments in favour of a Palestinian connection are that Palestinian groups have extensive experience of international terrorism; that Hizbollah was responsible for the last series of bomb attacks in Paris nine years ago; and that they are currently waging war against Yasser Arafat's successive agreements with Israel and exploded the bomb in Tel Aviv only the day before the Paris bombing.
It is noted that Yasser Arafat, their enemy number one after Israel, was due in Paris on the day of the attack (although he postponed his arrival until yesterday) and that his wife, Suha, is in a Paris hospital, having given birth to a daughter, Zahwa.
The arguments for a Bosnian Serb connection derive from the aggressive stance towards the Bosnian Serbs taken by President Jacques Chirac since he came to office and his insistence that the Western alliance must do more to repulse Bosnian Serb attacks on UN "safe areas".
Some, accepting speculation that a French plane or planes bombed the Bosnian Serb "capital", Pale, on Sunday, believe the St-Michel bomb was a direct reprisal.
One defence specialist, Jean-Christophe Ruffin, who worked in the office of the former defence minister, Francois Leotard, said that he favoured a Balkan connection for several reasons. He said that experts from former Yugoslavia had long warned France that "if you play games with the Balkans, you expose yourself to risk".
But he said that either the Serbs or Bosnian Muslims - disappointed that Mr Chirac had refused to take their side - could have mounted the attack, and probably had the network and means to do so.
The favourite theory by far, however, pinpoints Islamic terrorists who are opposed to France's (now diminishing) support for the military government in Algeria.
One suggestion is that those who planted the bomb could be from the same group held responsible for the assassination on 11 July of Imam Sahraoui in a central Paris mosque.
He was killed shortly after the failure of secret talks in Paris between representatives of the Algerian government and Islamic groups. The imam was a co-founder of FIS, the Islamic Salvation Front.
His assassins have not been found, and a range of suspects has been suggested, from the Algerian - or even French - intelligence services, to GIA, a more extreme Algerian fundamentalist group, or even dissidents inside FIS.
Another theory links the St-Michel bomb to the deaths of four GIA members when French commandos stormed a hijacked Air France jet at Christmas last year. Some believe that a special GIA cell was set up to mastermind a revenge attack.
A month ago, French police mounted a nationwide operation in which they detained dozens of people suspected of links with the GIA and the FIS. But, according to one source, the raids served only to demonstrate the extent of GIA networks operating in France.Reuse content