PARIS (Reuters) - Four suspected radical Islamists arrested in the Paris region this week on suspicion of plotting attacks in France were placed under formal investigation on Friday, legal and police sources said.

Three of the suspects are French and one is from Benin. Their ages range from 22 to 34, the sources said.

Two other suspects arrested in the same raid on a suspected Islamist cell on Monday were released.

Being placed under formal investigation in France means there is "serious or consistent evidence" pointing to likely implication of a suspect in a crime. It often, but not always, leads to a trial.

France has been on heightened security alert since January, when its military intervened in Mali to help repel al Qaeda-linked Islamist rebels who had seized control of the north of the former French colony.

Separately, a Muslim convert suspected of stabbing a French soldier in Paris in a religiously motivated attack was placed under formal investigation last month.

(Reporting by Gerard Bon; Writing by Catherine Bremer; Editing by Alistair Lyon)