A French counterterrorism source told CNN that the monitoring has been going on for "a couple of years" and includes the "national railway service SNCF, Paris public transport company RATP and airports such as Roissy Charles de Gaulle and Orly."
Samy Amimour, one of the three attackers in the November 13 assault at the Bataclan theater, had been a bus driver for RATP until October 2012.
Amimour, 28, was known to French security services since 2012 when he was placed under supervision after anti-terrorism authorities investigated an aborted attempt to travel to Yemen. He was charged for "activities in collaboration with a terrorist enterprise." But his travel to Syria and his return to France had not been recorded.
While the intelligence services knew about Amimour's radicalization, RATP did not, according to Elisabeth Borne, the President of RATP. Employers in France are not usually notified about staff who may have associations with terror suspects.
Complaints among unions
Ground and air transportation unions have been complaining about radicalization among some workers for several years.
"It started around late 2012, beginning of 2013 where some drivers and RATP employees refused to say hello to women or were praying inside the bus during their working hours," said Christophe Salmon, head of the union CFDT at RATP.
"According to the principle of secularism within the framework of the company, it is absolutely not authorized to show any religious sign," Salmon said.
Last week, French airport police conducted searches at several companies whose staff work at the airport -- some of them with access to the tarmac and aircraft, according to Christophe Blondel, Deputy to the Prefect of Airport police.
Paris airport authorities told CNN that these police searches specifically targeted three companies located at Charles de Gaulle: Air France Cargo, Servair and FedEx.
All of these companies confirmed the searches had been carried out but told CNN that none of their employees had been terminated. Blondel told CNN that so far there had not been any dismissals because the investigation is ongoing.
The police searches were conducted "airside" (beyond passport and immigration control) by 70 soldiers from the Air Transport Gendarmerie "in a locker room with 2010 lockers," Blondel told CNN.
To access "airside" at Charles de Gaulle, employees must have a special electronic pass-key.
Since January, "10 of these fobs have been taken from employees and 50 employees have been refused access" to the key, Blondel told CNN. "These individuals were suspected of being too radicalized to have access to airside."