French investigators have discovered weapons and a pledge of allegiance to the Islamic State group after charging three men with planning a terror attack, sources close to the inquiry said Thursday.
The men had not yet decided on a specific target, but had considered attacking a prison guard, another source had told AFP on Wednesday.
Two of the men are already serving prison sentences, and the letter claiming IS allegiance was found on the third man, one source said Thursday.
All have denied the claims after they were charged last Friday.
One of the men was Zakaria Chedili, 31, who was behind bars for spending six months in Syria in 2014 with a group affiliated to al-Qaeda, a source said.
The sources added that the seized weapons were "old", without further details.
"It was a project, we were not on a particularly high level of risk," Interior Minister Christophe Castaner said Thursday, after the arrests were reported by French daily Le Parisien.
The mastermind of the group was a 27-year-old convert to Islam who had been jailed for being an "apologist for terrorism", it said.
He had been transferred to at least three different prisons and it was during those transfers that he met the other two suspects.
The third man, aged 42, had been released from prison earlier this year, the paper said.
In recent months, two guards have been assaulted in prisons in the northwest of France, one by a prisoner convicted of terrorism and the other by a radicalised detainee.
Of some 71,000 prisoners in France, about 500 are either awaiting trial or already convicted for acts of Islamist terrorism.
A further 1,100 common-law prisoners are being monitored for radicalisation, according to official figures.