As in the best crime thrillers, detectives hunting the killers who gunned down wealthy Riviera property magnate Hélène Pastor discovered the devil was in the detail – in this case the shower gel.

It was the clue left in a hotel room that led police to Wojciech Janowski, Poland's honorary consul to Monaco and the dead woman's son-in-law, who appeared before a judge on Friday accused of ordering her murder.

Janowski, 64, a Cambridge-educated Polish businessman who had lived with Pastor's daughter Sylvia for 28 years – the couple had never married – confessed to being "implicated" in the killing of Pastor and her chauffeur. Sylvia Pastor, 53, who was arrested at the same time but later "totally exonerated", was said to be in a state of shock. Six others accused of involvement in the murder in May are being held in police custody.

Prosecutors say Janowski admitted giving his sporting coach €200,000 "as well as presents including holidays and a car" to organise the killing that he had been planning for "a long time".

"Janowski felt rejected by the rest of the Pastor family in general and by Hélène Pastor in particular. Janowski says he wanted to put an end to his wife's suffering at the hands of her mother, but Sylvia Pastor contests this entirely," the public prosecutor in Marseille, Brice Robin, told a press conference.

Two men, carrying a hunting rifle and a sawn-off shotgun, ambushed Pastor's Lancia Voyager car on 6 May as she left a Nice hospital where her son Gildo, 47, was recovering from a stroke. After firing twice into the vehicle and checking that both Pastor, 77, and her chauffeur of 15 years, Egyptian-born Mohamed Darwish, 64, had been hit, the men ran off.

Darwish died in hospital after four days and Pastor 15 days later. Before succumbing to her injuries, she told police she had no idea who might want her dead.

Robin said Janowski had obtained €8.4m from Hélène Pastor via her daughter in the past year. The prosecutor said he had no idea where that money had gone but it was not in Janowski's bank account.

He said Janowski had specified that he wanted Darwish dead "as a diversion and to muddle things up and make people question who was the real target" of the attack.

The cold-blooded assassination of Pastor, who boasted an estimated €20bn property empire in Monaco established by her grandfather, an immigrant Italian stonemason, shocked the principality where she was known as the "princess" and had links with the royal family.

Using images from video cameras, phone taps and satellite information from the suspects' mobile phones, police traced the two men who attacked Pastor to a €90 hotel room in Nice that they had rented and taken a shower in before carrying out the killing. The turning point in the inquiry was the discovery of DNA from one of the suspects in shower gel in the room.

Police later found a sports bag containing the guns used in the attack and a "large sum of money".

Detectives arrested Janowski for questioning about certain bank transactions on his account after discovering he had withdrawn €250,000 from an offshore account in Dubai.

Pastor's daughter Sylvia, known as "Sisi", and son Gildo received generous allowances reported to be up to €500,000 a month and lived in luxury apartments owned by their mother.

Janowski, the Polish honorary consul in Monaco since 2007, is known in the principality for his charity work, particularly with the organisation Monaco Against Autism (MONAA), which he co-founded and which boasts Princess Charlene of Monaco as its honorary president.

He was given the National Order of Merit of the French Republic by Nicolas Sarkozy in 2010 for his charity work and has a master's degree in economics. In the past he ran a chain of hotels and casinos in the principality.

In the end, the killing of Hélène Pastor and her chauffeur turned out to be as the French press had suggested from the beginning – a "crime crapuleux", a criminal act sparked by a motive as old as the Monaco hills: money.