HEARTENING proof of the march of democracy in Chile comes with the arrests of two former security officers who have long been implicated in the murder of Orlando Letelier. Mr. Letelier, a onetime official under Chilean President Salvadore Allende Gossens, was killed by a car bomb in Washington in 1976. Evidence points to Gen. Juan Manuel Contreras Sepulveda, the head of the secret police during the military rule of Gen. Augusto Pinochet Ugarte, and Col. Pedro Espinoza Bravo, who served under Contreras, as the masterminds of the plot. Six underlings have been convicted in the US in connection with the Letelier murder, but for years it seemed unlikely - despite repeated urgings by Washington - that Contreras and Espinoza would ever be prosecuted. But now, just days before the statute of limitations would have expired, the two officers have been detained on the orders of Supreme Court Justice Adolfo Banados, who was placed in charge of a special investigation by democratically elected President Patricio Aylwin Azocar. Justice Banados had a strong human rights record as a lower-court judge before President Aylwin elevated him to the high bench. Between 1973, when Pinochet seized power in a bloody coup, and 1978 the military government was responsible for the death or disappearance of thousands of political opponents, as described in a report issued by a presidential commission earlier this year. Most of the violations are covered by an amnesty law Pinochet signed in 1978, but, under US pressure, the Letelier case was excluded from the bill. The Letelier case has high symbolic importance in Chile as well as the US, for it is one of the few cases from the darkest days of Pinochet's repression that can be prosecuted. Public demands for justice are loud. However, elements in the military and the political right will also work to undermine the case by tarring it as a product of American meddling. The US can help undercut that claim by restating its hands-off support of Chile's justice system under Aylwin's administration.