Paris As France asks itself whether it could have done more to prevent Islamist gunman Mohamed Merah shooting dead seven people in a killing spree that shook the nation, there is one question that refuses to go away: how did he obtain so many guns.
The size and nature of the arsenal amassed by Merah - who stockpiled at least eight guns including a Kalashnikov assault rifle and an Uzi machine pistol - has focused attention on the easy availability of illegal weapons in France and their growing use in ultra-violent crimes.
As an angry online reader of the daily Le Figaro newspaper put it: "How was he able to buy all these guns, like one buys yoghurts, when he was under the surveillance of the DCRI (the French intelligence agency)?"
Just weeks after France tightened gun laws which were already among the strictest in the world, the issue has blown into the political debate ahead of an April-May presidential election.
On Friday, Francois Bayrou, a centrist candidate who is supported by around 12 percent of voters in opinion polls, demanded a crackdown.
"How is it possible for a man with his background, a young man like him, to get hold of guns used in wars," he asked the France 2 TV channel, adding that it was time to devise a plan to choke off the flow of illegal arms.
"The availability of guns must be placed at the centre of our national concerns," he said.
Some of the weapons Merah obtained are more typically found on a battlefield.
His arsenal reportedly included at least three Colt .45 pistols, the U.S. army's sidearm of choice during the Second World War, a 9mm Sten submachine gun, a Kalashnikov assault rifle, a pump-action shotgun, an Uzi machine pistol, as well as a Colt .357 Python revolver.
AN UNLIKELY GUN OWNER
Merah's killing spree in the southern city of Toulouse has shaken up the presidential battle between conservative incumbent Nicolas Sarkozy and Socialist frontrunner Francois Hollande and has turned upside down a campaign that was dominated by economic concerns.
Merah's profile did not, at first glance, suggest someone capable of amassing an illegal arsenal that would have cost at least 10,000 euros ($13,000) to purchase, according to gun experts.
Just 23 years old, he was an unemployed panel beater and was claiming state benefits worth only 475 euros ($625) a month.
He had, however, been involved in petty crime and appears to have used the proceeds to quietly build up his arsenal.
Francois Molins, a Paris-based prosecutor, said that Merah had told how he had funded his collection before he was killed by a police sniper on Thursday.
"He explained that he got his hands on all these weapons because he had pulled off break-ins and burglaries that provided him with money to buy guns and ammunition," Molins said.
TOUGH GUN LAWS
Already tough French gun laws were tightened further earlier this month with the approval of longer jail terms and larger fines for anyone caught with an illegal firearm.
Amateur marksmen and game hunters must meet stringent criteria to obtain a gun licence including registering themselves with the authorities, proving they have no criminal record, and passing a psychological evaluation. There is also a blacklist of some 18,000 people banned from owning a gun.
The government says there are at least 7.5 million guns in legal circulation in the ownership of the state, amateur gun enthusiasts and hunters. Nobody knows how many illegal guns are in circulation, though experts put the total number of both legal and illegal guns at between 10 and 20 million in a nation of 65 million people.
Merah's frequent brushes with the law would have made it impossible for him to have legally acquired any of his weapons, gun experts say.
"There is no way he could have got these guns legally with his record," Jean-Jacques Buigne, president of the French union of amateur gun owners told Reuters. "It is out of the question."
Most of the guns he got hold of, such as the Uzi machine pistol, are in any case banned in France. The guns he used would have traced their origin back to World War Two, Eastern Europe or a more recent conflict in the Middle East, experts say.
Although the French public appears to have been shocked by Merah's arsenal, experts say getting hold of such guns is relatively easy on the black market if someone is patient and has money.
"There are people who are in the market for this kind of thing, it's the law of supply and demand unfortunately," said Francois Thevenot, secretary of the French magistrates' trade union.
Experts say a Colt .45 pistol of the kind Merah used to execute his victims can be bought for 600 euros ($800) legally and around 1,000 euros ($1,300) illegally. Kalashnikovs go on the black market for anything between 1,500 and 3,000 euros ($2,000 to $4,000).
Nor, adds Buigne, is it surprising that Merah was able to assemble such an arsenal.
"It is not odd that he fell through the cracks because we live in a free and liberal society not a police state. You need a court order to conduct a search."
Though bloody rampages such as Merah's remain rare in France - the last time a lone gunman killed so many people was in Nanterre in 2002 - police say the use of illegal arms in violent crime is on the rise. In 2010, police confiscated 2,710 guns, 79 percent more than in 2009.
The southern port city of Marseille, where rival drug gangs sometimes settle their scores with Kalashnikovs, is one of the places worst affected in France.
In 2011 alone, gang warfare there killed 16 people and wounded 13, and Kalashnikovs are increasingly the weapon of choice, according to David-Olivier Reverdy, a spokesman for the Unity police union.
"In Marseille and the surrounding area almost all the score-settling is carried out using weapons used in wars," he told Reuters. "If you don't have a 'Kalash' you're a bit of a loser."
(Additional reporting by Marion Douet in Paris and Jean-François Rosnoblet in Marseille; Editing by Giles Elgood)