The woman in her 40s was killed at Bel-Air station last Saturday afternoon.

Her husband and son were with her as she made a late exit from the carriage, and the door shut on her coat.

Unable to extricate herself, she was pulled along the platform as the train moved off.

There have been three deadly accidents on the Paris underground in the last two weeks.

A 14-year-old American girl was killed when she fell in front of a regional train at Cité-Universitaire station, and two homeless people who had ventured onto the lines in a drunken state were hit by a train at Gaieté.

Colleagues of the driver in the Bel-Air accident in eastern Paris were angered by his being held in detention on Thursday night, and then being placed under judicial investigation for involuntary homicide.

With 15 years experience and a clean record, the man tested negative for alcohol and drugs. If convicted, he could face five years in jail.

"Tragic incidents happen from time to time, but we have never seen a colleague being placed under judicial investigation like this," said Jean-Christophe Delprat of the Force Ouvrière (FO) union.

"The police came in force to pick him up in front of his traumatised wife and children. They're treating him like a criminal."

On Friday morning, drivers on Line 6 - the metro line of Bel-Air - stopped work in solidarity with their colleague, who has now been released from custody. Trains on the line began rolling again in the afternoon.

The criminal investigation, and a concurrent technical one, will seek to establish the exact circumstances of the woman's death, and if systems or machinery failed to function properly.

The train was an old model, dating from the 1970s, with a manually operated latch for opening the doors.

Normally the driver should be alerted by an alarm if a coach door fails to shut fully. But union officials told Le Parisien newspaper that there was "give" in the system, and sometimes a door with something caught in it could register as shut.

There are several cases a year of passengers being trapped in a metro door and dragged along the platform, according to Bastien Berthier of FO.

"But in the vast majority of cases the drivers are alerted by the shouting or by an alarm and stop the train in time. But it all can hinge on a fraction of a second," he said.

The Paris metro registers around 60 deaths, euphemistically known as "Serious Passenger Accidents", every year. Nearly all involve people taking their lives.