Amsterdam/Duisburg, Germany  - Giovanni Strangio had for months lived a modest life in the anonymity of Amsterdam until a careless error put police on his trail.

A German environmental sticker issued for a Peugeot car registered in the name of a 48-year-old contact person in Frankfurt led investigators to the suspected Mafia hit-man.

Late on Thursday evening Strangio, his wife, their young son and another man named Francesco Romeo were taken into custody at an apartment in the Amsterdam suburb of Diemen.

Strangio was taken by surprise when heavily armed Dutch police burst into the living room. Romeo, his suspected accomplice in an execution-style killing of six men in Duisburg 18 months ago, was in bed.

It was a huge success for a combined operation involving Dutch, German and Italian police looking for the perpetrators of one of the most brutal crimes committed in Germany for a long time.

Germany's federal police had been searching the globe for Strangio ever since he was reported to have killed six members of a rival clan of the Calabrian mafia 'Ndrangheta with shots to the head.

His accomplice on that rainy night of August 15, 2007, was believed to be Romeo, another Mafia member.

Police at the time spoke of a spectacular execution in which the killers used a rapid fire Beretta-93-R pistol as their victims were sitting in a car outside a Duisburg pizzeria.

Investigators suspect Strangio and the other man carried out the slayings, even if investigations into the crime are still going on.

Police found what they believe are DNA traces of Strangio and powder burns from the murder weapon in the getaway car, a Renault Clio. DNA traces, thought to be from his co-accused, were discovered on the passenger seat.

Police are conducting tests to see if the DNA samples match.

Surveillance cameras at a company near the pizzeria showed showed the getaway car had been at the scene several days before the killings. Police had also determined the Strangio had attempted to buy ammunition for the murder weapon.

Telephone wiretaps led investigators to Amsterdam soon after the killings, Duisburg police chief Holger Haufmann said on Friday.

When three of Strangio's sisters visited the Dutch city in November last year, police moved in and arrested Mafia boss Giuseppe Nirta.

Nirta, who is married to one of Strangio's sisters, had also been sought by Italy for more than a decade to serve a sentence of nearly 15 years for drug trafficking.

Police found the vital clue, the car environmental sticker, in Nirta's Amsterdam apartment. The investigators traced it to the contact man in Frankfurt and kept on his heels.

They followed him to Amsterdam where they found Strangio's hideout after the 48-year-old met with Romeo.

Police were impressed by the discipline with which Strangio shielded himself from the outside world. During the entire observation period, he left his apartment only once, and then only to step briefly outside the door.

The rest of the time he was ensconced in the flat with his brother-in-law and the rest of the family.

Strangio let his hair grow long and dyed it blond. He also wore a baseball cap to aid his disguise. This made it difficult for Italian and German mafia specialists to identify him on photos.

It was a different case with his wife, who was easily identifiable on a photograph. Police also knew the Strangios had a three-year-old son. The couple they were observing also had a small child.

Then police moved in and arrested her husband and Romeo. The mother and child were later released. Unwittingly they had helped police nab the suspected killers. (dpa)