Police in France investigating the sabotage of high-speed rail lines hours before the Paris Olympics' Opening Ceremony have asked the U.S. FBI for help, two sources with direct knowledge of the French inquiry said, Paralel.Az reports citing Reuters.

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."