Who could have attacked France's high-speed rail?

For France's Interior Minister Gérald Darmanin - the man with the task of securing the Paris Olympics - the sabotage attacks on the high-speed rail network will have come as a blow.

He has vowed the attackers will all be quickly arrested, but so far he has not indicated who might be to blame.

Sports Minister Amélie Oudéa-Castéra promised that the culprits were not going to spoil the party, but they struck the TGV network hours before the opening ceremony - causing chaos for travellers and exposing the vulnerability of a symbol of France's technical prowess.

Caretaker Prime Minister Gabriel Attal has advised caution in drawing conclusions, but said those behind the attack clearly had a good understanding of what would cause most damage.

Suspicion fell immediately on ultra-left radicals, from security sources briefing French media, but there has been no claim of responsibility from any source.

So far all we know is that the methods used to set fire to critical optical fibres and other cables in ducts along the rail network in the early hours of Friday were reminiscent of previous attacks by the extreme left.

When cable ducts were set alight beside railway lines near Hamburg in Germany last September, an anonymous claim appeared on a left-wing website condemning "capitalist infrastructure".

That is inconclusive, of course, because the broad nature of the French attacks suggests a degree of co-ordination across four distinct regions that would not normally be associated with the extreme left.

But whoever did target the rail lines stretching out of Paris in the early hours of 26 July, it was clear they had the Games in their sights.

The big TGV arteries to the north, east and west were all choked off and the high-speed line to the south-east would have been brought to a halt too, but for an alert crew of engineers who by chance spotted a team of saboteurs in "vans".

Regional forces are collecting evidence under the overall command of the national police, the national gendarmerie as well as the anti-terrorist SDAT. Their biggest hope may be in tracking down the failed saboteurs who fled the scene near Vergigny, apparently leaving their intact incendiary devices behind.

There have been attacks on French railways before, including one in January 2023 east of Paris.

Another incident has only just emerged back in early May 2024, on the high-speed line to the south, just outside Aix-en Provence.

It is that attack that bears most similarity to Friday's sabotage, because it reportedly took place on the day the Olympic flame arrived by ship in the southern port of Marseille. So far no arrests appear to have been made.

Even though it was a botched attempt, reportedly involving makeshift petrol-bombs, France's security services will be looking at potential links to that attack.

Earlier this year, the interior minister warned of an extremely high "external" threat, potentially of the type of jihadist attack that was inflicted on the Crocus City Hall in Moscow in March.

France has fallen victim in recent years to a wave of deadly jihadist attacks, but none resemble the acts of sabotage inflicted on the rail network. Friday's incidents caused misery for hundreds of thousands of travellers, but no bloodshed.

Suspicion will inevitably fall on Russia too, a country in the grip of a full-scale invasion of its neighbour Ukraine, and one that has engaged in a high-profile campaign of disinformation against France.

Pro-Kremlin social media accounts have shared a video smearing the Paris Games, ridiculing the quality of water in the River Seine and attacking President Emmanuel Macron and Paris Mayor Anne Hidalgo.

Mr Macron is loathed in Moscow because of his outspoken support for Ukraine.

Although Russia has always denied interference, French officials suspect the hand of Moscow in a series of recent incidents aimed at destabilising the French capital. From red hands daubed on the Holocaust Memorial to graffiti on buildings suggesting their balconies might collapse.

Only this week, a Russian was arrested in Paris on suspicion of planning to organise acts of "destabilisation, interference and spying". The Kremlin says media reports on the man have been "quite curious" but says it has not been directly told about the arrest.

But none of that necessarily implicates Russia in Friday's co-ordinated attack on what caretaker Prime Minister Gabriel Attal calls "nerve centres" on France's high-speed railway network.

Because whoever was behind the sabotage knew exactly where to cause maximum disruption. Russia might not have that kind of reach in rural France.

The head of state-owned rail company SNCF, Jean-Pierre Farandou, said the saboteurs had focused on intersections that would have caused the most serious impact.

The arson attack at Courtalain cut off two high-speed lines on the Atlantic artery, one that headed west towards Brittany and another towards Bordeaux in the south-west. The eastern attack knocked out high-speed lines to Metz in one direction and Strasbourg in another.

One French security expert, Romain de Calbiac, told the BBC's Newshour programme that the attack was remarkably well-planned.

"The French security forces and the entire intelligence community here is very concerned that they might have received internal help from people working or people partnering with the railway network in France," he said.

"Another option would be that this information came not from inside sources, but potentially from foreign states with a knowledge of how the French network works."

Earlier this year, SNCF highlighted an increase in the trend for attacks on the rail network and said it was constantly on the look-out for acts of sabotage, "particularly in the run-up to the Olympic and Paralympic Games".

Although the company said it had detected all the attacks on its systems, it was only able to prevent one from causing significant damage, and that was a stroke of luck.

"Today should have been a party," said Jean-Pierre Farandou. "All that is ruined."