Investigators called in the FBI after media organisations, including Reuters, received an email the police believe may have been sent by the perpetrators. It set out a rambling list of grievances but did not explicitly claim responsibility.

The email, signed by "an unexpected delegation", was sent from a riseup.net address, a Seattle-based collective that says it provides "communication and computer resources to allies engaged in struggles against capitalism and other forms of oppression".

The saboteurs targeted four high-speed TGV lines heading into Paris with incendiary devices, causing travel chaos hours before a global audience tuned in to follow the parade of Olympic athletes along the River Seine on July 26.

There have been no arrests. French officials suspect domestic far-left groups, but have not ruled out foreign involvement.

As riseup.net is U.S.-based, French police have asked the FBI to push that organisation to identify the email account-holder, the two sources told Reuters.

The FBI declined to comment.

Riseup did not respond to an emailed request for comment.

The FBI would need to subpoena Riseup to hand over the details, but Riseup would be unlikely to comply.

"We will actively fight any attempt to subpoena or otherwise acquire any user information," it says on its website. "We do not share any of our user data with anyone."

In a 2020 interview with Belarusian anarchist group Pramen, Riseup said it had never complied with a foreign legal request.

"We regularly get legal requests from all around the world. We are not required to respond, and so our policy is to not."

However, the FBI has previously served warrants on Riseup.

In 2012, FBI agents seized a server used by Riseup as part of a probe into bomb threats. In 2017, Riseup begrudgingly complied with two FBI warrants seeking the identity of accounts involved in extortion.

The author of the email railed against France's arms, energy and nuclear sectors, and criticised a TGV network as only benefiting "a few privileged people". They said the impact of the attacks - "spoiling tourists' trips or disrupting holiday departures" - was minimal when compared with the legacy of a transport system that has "always been a means for the colonisation of new territories".

FOREIGN FINGERPRINTS?

Both sources said French investigators have yet to find firm evidence of foreign involvement.

One source said police are looking for a possible link to a 2023 hack of the Polish rail system, which Polish intelligence initially believed was carried out by Russia.

In December, a Polish court sentenced 14 citizens of Ukraine, Belarus and Russia for spying for Russia. The operatives were monitoring rail transports of arms for Ukraine via Poland and planning to disrupt them.

France has accused Russia of waging a concerted disinformation campaign to sow chaos ahead of the Olympic Games, now in their second week. French police arrested a Russian man in July suspected of planning to destabilise the Olympics.

The Russian Foreign Ministry did not respond to a request for comment lodged after the sabotage incident.

Police have yet to establish a link to a similar sabotage of France's telecoms network two days after the TGV attacks, the sources said. Although both involved incendiary devices, one source said the explosive materials used were different.

One line of enquiry relates to explosive material found on a TGV line near Marseille on May 8 - the same day the Olympic torch arrived in the city. The explosive material was almost identical to that used in the July 26 attacks, the source said.

The prosecutor's office in Aix-en-Provence confirmed it was investigating, but declined to comment further.

French police are investigating former employees of state-owned rail operator SNCF, as well as third-party contractors, but one of the sources said much of the knowledge needed to undertake the TGV sabotage can easily be found online.

French police are also in talks with Italian, German and Spanish counterparts as the attacks may have been coordinated with anarchist groups from abroad, the source added.

Far-left and anarchist cells typically oppose the state and capitalism, and are increasingly incorporating environmental concerns into their ideologies, French officials say.

They have proven hard to penetrate, the two sources said.

Their members often live off-grid and are paranoid of outsiders. They shun mainstream communication channels and practice impeccable digital hygiene. As their attacks are generally non-lethal, they are less of a priority for security services predominantly focused on deadly Islamist militants.