The families of the victims have called for a fresh investigation into the killings, which took place in 1980, but the chief prosecutor Manuel Cordova said the time lag made it legally impossible to revive the case.

Mr Cordova also said it would be the authorities' final word on the matter.

Shocking deaths

The three Roman Catholic nuns and a young missionary were attacked by a group of soldiers on a country lane in El Salvador in 1980. The women were raped and shot dead.

The BBC's Central America correspondent says the crime was one of the most shocking episodes in El Salvador's civil conflict, and prompted the United States Government to cut off military aid.

Five soldiers were later convicted of the killings, which the Salvadoran Government continues to describe as acts of common crime.

Relatives of the murdered women who have been pressing for a proper investigation for more than 20 years are convinced that high-ranking military officers ordered the attack because of the victims' alleged links to left-wing guerrillas.

This theory was said to have been confirmed earlier this year by the jailed soldiers themselves. Their reported confessions have renewed public interest in the case, strengthening calls for a fresh investigation.