The imprisoned son-in-law of a murdered Monaco billionaire goes on trial Monday with nine other people suspected in the brazen daylight killing of the heiress, a case which prosecutors say bears all the hallmarks of a sordid crime thriller.

Hélène Pastor was leaving a hospital in Nice on the French Riviera after visiting her son on May 6, 2014, when a gunman shot her and her driver in their car with a sawed-off shotgun.

The chauffeur died a few days later and Pastor fell into a coma, but she woke up and was able to tell investigators: "I'm afraid, I want to see you again because I have more to tell".

Before she could, she died of her injuries on May 21.

Police quickly identified two men suspected of carrying out what they now believe was a contract killing orchestrated by Wojciech Janowski, Pastor's Polish son-in-law.

After initially admitting to ordering the killing to get his hands on Pastor's fortune, estimated at 12 billion euros ($14 billion), Janowski later retracted his confession, saying he had misunderstood investigators' questioning.

The suspected killer, Samine Said Ahmed, 28, is also on trial and has denied the charges. Al Hair Hamadi, 35, is accused of acting as a lookout.

Prosecutors say the two men, originally from the Comoros islands and living in Marseille's rough northern districts, left behind a multitude of clues.

Video surveillance cameras in Marseille caught them buying mobile phones and boarding a train for Nice, where their DNA traces were found on a bottle left in a hotel room.

They are also seen waiting for Pastor opposite the hospital, where they arrived by taxi after unsuccessfully trying to buy a scooter for their getaway.

"It's the first time we've seen killers use a taxi to reach the scene of a crime -- they did everything you're not supposed to do," a source close the inquiry said.

Mother-in-law 'a monster'

Pastor had inherited a huge real estate and construction business set up by her Italian grandfather Jean-Baptiste Pastor, a stone mason who moved to Monaco in 1880.

Phone records led to several suspected accomplices who will also be on trial in Aix-en-Provence, just outside Marseille, with the hearing expected to run until October 19.

Janowski, 69, has been married 28 years to Pastor's daughter Sylvia, and was a businessman and Poland's honorary consul to Monaco before being stripped of the title after he was charged.

A graduate of the University of Cambridge, he headed a nanotechnology firm and an oil business, and was said to be involved in numerous charities in the principality.

But prosecutors say his business ventures were not as successful as he claimed, and he was hoping the inheritance money would help him recover even though he was already receiving part of the 500,000 euros Pastor gave her daughter each month.

A month after the murders, Pascal Dauriac, 49, Janowski's personal trainer, admitted to organizing the hit, using his brother-in-law to recruit the two killers.

"Janowski tricked me... He said his mother-in-law was a monster," Dauriac said.

He also claimed Janowski himself had scouted out the hospital and wanted to have Pastor's son killed too.

Janowski allegedly spent 250,000 euros ($291,000) to hire contract killers.