The United States had filed an appeal in November, days after judges refused an extradition request on the grounds that 68-year-old George Wright had become a Portuguese citizen.

Wright, arrested on September 26, had been living in Portugal for 20 years at Almocageme, a village some 30 kilometres (18 miles) from the capital.

He had changed his name to Jose Luis Jorge dos Santos and was married with two adult sons.

US authorities had been searching for Wright since his escape in 1970 from a New Jersey prison, where was jailed for a 1962 murder.

For two years after his escape, Wright disappeared into the Black Liberation Army, a violent group of African American communists.

Then on July 31, 1972, he was among a group of five people who hijacked a Delta flight from Detroit to Miami.

They demanded a $1 million ransom for the passengers. When that was paid, they forced the plane to fly to Boston where it refuelled and took another pilot hostage, then crossed the Atlantic to Algeria where they sought asylum.

Although the plane and money were seized by the Algerians and returned to the United States, the hijackers were only briefly detained.

In May 1976, French police captured four of the gang — but not Wright.

He had managed to acquire his new identity in Guinea Bissau, a former Portuguese colony which granted him political asylum in the early 1980s.

Wright acquired Portuguese nationality a few years later, following his marriage.