Switzerland's highest court on Tuesday sentenced a family of Swiss engineers accused of helping Libya in its failed efforts years ago to build up a nuclear weapons programme.

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But Friedrich Tinner, 70, and his two sons Marco and Urs were free to leave the Federal Court of Justice in Lausanne because of the length of time they had already served in detention and a plea bargaining agreement. 

Urs and Marco Tinner received sentences of 50 months and 41 months respectively, while Friedrich was given a 24-month suspended sentence for offences under the War Material Act.

The case began in the 1990s, when the Tinners started working with the global nuclear smuggling network of Abdul Qadeer Khan, the so-called father of the Pakistani atom bomb, who supplied Libya with nuclear weapons technology.

Swiss prosecutors launched a case against Marco, 43, and Urs, 46, in October 2004 and against their father the following year.

After their arrests in 2004 and 2005, the brothers spent three and four years in jail respectively awaiting trial, while their father was incarcerated for nearly two years.

Before sentencing, the defendants refused to talk about their collaboration with the CIA, which began when Urs contacted the US intelligence agency in 2003, the court heard.

The information they supplied pointed authorities to a German cargo ship that was stopped in the Mediterranean en route to Libya, media reports said.

Five containers filled with sensitive material were seized, effectively blocking Libya's nuclear ambitions.

The Tinners said they had not spoken to Swiss authorities because "the affair was in good hands", in reference to the United States.

The strange case was the subject of a book published last year by two US journalists, Catherine Collins and Douglas Frantz, called "Fallout", which tracks the way the United States secretly penetrated Khan's network to prevent Libya and Iran from obtaining nuclear secrets.