Story highlights Marine Le Pen's chief of staff taken into custody is subject of judicial inquiry

Le Pen, of the far-right National Front, is one of the front-runners in France's presidential race

Paris (CNN) Marine Le Pen's bid to become French president has become embroiled in controversy after her chief of staff became the subject of a formal judicial investigation into whether she held a fake European Parliament job.

Catherine Griset and Le Pen's bodyguard, Thierry Legier, are alleged to have been paid for non-existent jobs at the European Parliament.

Griset was put under formal investigation regarding allegations of concealment of breach of trust. She was released from custody as the judiciary began an investigation on the parliamentary assistants of the far-right National Front, the Paris prosecutor's office said.

It was not clear late Wednesday whether Legier was in custody.

Le Pen, leader of the National Front, initially admitted they had been paid while not working, the European Anti-Fraud Office (OLAF) said. She later denied having said so.