A French investigating judge is due to decide later on Wednesday whether to formally investigate Pavel Durov, the Russian-born head of Telegram, who was arrested as part of a probe into organized crime on the messaging app.
Durov's arrest after arriving at an airport near Paris on a private jet on Saturday evening has focused attention on criminal liability for app makers and sparked a debate about where free speech ends and law enforcement begins.
With nearly 1 billion users, Telegram is especially popular in Russia, Ukraine and the former Soviet republics.
A formal investigation in France does not necessarily mean a person is guilty, and it does not necessarily lead to a trial, but it indicates that a judge believes there is sufficient reason to proceed with an investigation.
Investigations can take years before going to trial or being shelved.
If Durov, who has been in police custody since his arrest, is subjected to a formal investigation, the judge will also decide whether to hold him in pre-trial detention.
One factor they will consider is whether he was likely to attempt an escape.
A source at the Paris prosecutor's office said an update on the investigation could be announced later on Wednesday.
At this stage, the overall investigation is directed at an unspecified number of people.
According to prosecutors, the charges focus on alleged complicity in crimes including operating an online platform enabling illegal transactions, possession of child sexual abuse imagery, drug trafficking, fraud, refusing to provide information to authorities and providing encryption services to criminals.
Prosecutors did not say what crimes Durov himself is accused of.
Durov's French lawyer did not respond to repeated email and phone requests for comment from Reuters.
President Emmanuel Macron said his arrest was not politically motivated but it caused further damage to already strained diplomatic ties between Russia and France, which has been helping Ukraine in its fight against Russian aggression.
Durov was taken into custody at 8pm on Saturday and his detention could last for 96 hours, or up to four days, until a judge decides whether to open a formal investigation.
